1Introduction

Are there exotic sources of radio impulses in other galaxies?
Do undiscovered planets exist in our solar system?
What happens to supermassive black holes when two galaxies collide?
How correct was Einstein’s theory of relativity?
Strangely enough, all of these questions can be addressed through the study of stars that
extend no larger than a few tens of kilometres across. This thesis deals with a wide
range of topics—all, however, are endeavours motivated by the broad area of researchcommonly termed “Pulsar Astronomy”. This introduction and the following chapters aim
to communicate the rich body of scientific research related to pulsar astronomy and its
related technical requirements, even if studying pulsars is not always the central aim of
the research. In this chapter we provide a context to understand the advances madein this thesis. The first section introduces pulsars and relevant properties of the stars
themselves. The second section gives an introduction to using pulsars as tools to study
other physical phenomena, and the third outlines the relationship that pulsar studies haveto the exploration of gravitational wave sources, in particular binary supermassive blackhole systems. The fourth section reviews the specialised technical systems designed to
study pulsars, and how they are being used to search the vast frontier of undiscovered
celestial transient radio emitters.

1.1 Pulsars
Pulsars have only been the subject of observation since late 1967, when highly periodic
pulses were recorded and recognised as a celestial phenomenon by Hewish et al. (1968).
The prediction of Baade & Zwicky (1934) stated that compact objects should exist at the1

2

Chapter 1. Introduction

end of the life of massive stars, leading Gold (1968) and Pacini (1968) to draw a connection
between this prediction and the observations of Hewish et al. In the just over forty years
since that pivotal discovery of the first pulsar, a great deal has come to be understood
about these objects. The standard model of pulsars is that they are the compact remnantsof massive stars, the core of which has collapsed and received a high-velocity kick as thehost star undergoes a supernova. The core forms a dense neutron star, counteracting its
own gravitation with the degeneracy pressure from the tightly packed neutrons that makeit up. The drastic birth conditions of pulsars leads them to have extraordinary properties,
with magnetic field strengths, densities, and energy levels that would be impossible (or,at the very least, highly perilous!) to replicate on Earth. We are able to observe andstudy radio emission from pulsars due to the highly collimated beams of electromagnetic
radiation that form along a pulsar’s magnetic axis (Fig. 1.1). Due to misalignment between
the magnetic and rotational axis of a pulsar, these beams can sweep across Earth once
per rotation of the star, causing a “lighthouse” effect for an observer that manifests itself
in radio light as a train of periodic pulses. Pulsars have exceptionally stable rotational
periods (e. g. Verbiest et al. 2008), making them highly accurate astrophysical clocks,
a property which has been exploited to build the bulk of scientific advances in pulsar
research.

1.1.1

Pulsar Energetics and Physical Parameters

At present just under 2000 pulsars have been discovered.1 While a definitive physicalmodel for where and how radio pulsar emission is generated has yet to be established,
the known pulsars provide us with a body of radio-wavelength properties and allow us
to infer basic electromagnetic properties of pulsars. First, regardless of luminosity or
fluctuations in the intensity of a pulsar beam (e. g. the nulling pulsars of §1.1.3), allpulsars observable in radio emission have a strongly periodic signal, with an observed
period P . In addition, pulsars are observed to exhibit a small lengthening of their periods
when observed over a sufficiently long timescale. This period derivative, Ṗ , is attributable

to loss of rotational kinetic energy, which powers e. g. magnetic dipole radiation (Pacini
1968, Ostriker & Gunn 1969), pulsar winds (consisting of charged particles streaming fromthe pulsar; Gold e. g 1969), electromagnetic emission in γ- or X-rays, with a small fractionof this energy emitted in the observed radio signal. For the purposes of this discussion,
we employ a simple pulsar model in which the rotational energy loss implied by the spin1

As queried in the most up to date catalogue first published by (Manchester et al. 2005); see
http://www.atnf.csiro.au/research/pulsar/psrcat/ .

1.1. Pulsars

3

Figure 1.1 A toy model of pulsar radio emission as described in the text, adapted from
Lorimer & Kramer (2004). The magnetosphere of the pulsar may co-rotate within the lightcylinder, beyond which co-rotation would imply superluminous velocities. The radio beam
is shown to originate from both magnetic poles along the open magnetic field lines. To
the right, we indicate the signal seen by observers viewing the pulsar from three different
angles.
down observed in a pulsar is due entirely to magnetic dipole radiation.2 We can calculatea pulsar’s spin-down luminosity (Ė) by relating the rotational kinetic energy loss to thepulsar’s magnetic dipole radiation. Both of these quantities relate to the pulsar’s period
and period derivative, giving:Ė =4π 2 I Ṗ
2  2 π 4=(|m| sin α)2
P3
3c3 P

(1.1)

(Lorimer & Kramer 2004), where |m| is the dipole’s magnetic moment, I is the pulsar’s
moment of inertia, and α is the angle between the magnetic and rotation axes. From thiswe can determine the evolution of the rotation period of the pulsar with time:
Ṗ =

2

dP
8π 2
=
· (|m| sin α)2 · P −1 .
dt
3Ic3

(1.2)

The pulsar is assumed here to exist in a vacuum, ignoring other energy dissipation effects: mostnotably, those related to the pulsar magnetosphere. However, this model is the most basic (derived fromclassic electromagnetics) and is appropriate for most practical pulsar applications. It furthermore allows
the derivation of a straight-forward estimator for characteristic age and magnetic field strength of pulsars.
No comprehensive model to describe the behaviour of all pulsars has yet been established, but for a recent
review of such models, see Lyubarsky (2008).

4

Chapter 1. Introduction

We now adopt a commonly assumed value for the pulsar’s moment of inertia of I =1038 kg m2 (Lorimer & Kramer 2004). On integrating equation 1.2 over time and assumingthat the pulsar’s period when it was formed is much less than its currently observed period,
we can define a characteristic age of the pulsar by the simple relation:
τc =

P
,
(n − 1)Ṗ(1.3)

Where n represents the braking index of the pulsar, which is defined by the energy loss inthe system, and relates the evolution of the first frequency derivative of the pulsar’s spin
to its period (ν̇ ∝ ν n ). For the pure magnetic dipole braking scenario used here, n = 3.
Under the same pure magnetic dipole radiation scenario, the magnetic field strength
at a distance r relates to the magnetic dipole moment as B = 2|m| r −3 . Equation 1.2
can thus be used to calculate the pulsar’s surface magnetic field strength at the pulsar’s
radius, R, also in terms of observables P and Ṗ :Bsurf = sin α
In practice we use 3.2 × 1015

α = 90◦ .



3Ic3
P Ṗ
8π 2 R6

0.5

.

(1.4)

p
P Ṗ Tesla, which is essentially a lower limit to Bsurf , setting

The surface magnetic field, rate of kinetic energy loss, and characteristic ages of known
pulsars are well-summarised by the so-called “period-period derivative” diagram. This isshown for all pulsars with currently known P and Ṗ in Figure 1.2. The “graveyard” inthis diagram indicates combinations of P and Ṗ for which in various models, there is aninsufficient supply of energy to trigger the positron-electron pair production that gives
rise to radio emission (here we indicate the line given by the polar-gap models of Chen
& Ruderman 1993). The different types of pulsars indicated in this diagram are further
discussed in §1.1.3. It is presumed that as pulsars evolve, their periods will slow at a ratedepending on their magnetic field strength until they can no longer support the productionof a radio beam, crossing the death line and becoming unobservable at radio wavelengths.

1.1.2

Basic Radio Observables and the Interstellar Medium

When searching for pulsars there are several basic observable parameters: a pulsar’s period,
pulse width, and its dispersion measure. To provide the reader with an idea of the typicalvalues exhibited by pulsars, in this section we summarise them and introduce the effects
on pulsed signals caused by the interstellar and intergalactic media. Pulse intensity (in1.1. Pulsars

5-8

Binary systems
RRATs
Magnetars
XDINS

-1

log[Period derivative (s s )]

-10

-1

29 J
0
1

s

-12

-14

-16

-18

r

10 ky

10 8 T

r

1 My

100

10 7 T

Myr-20‘‘Pulsar
Graveyard’’

-1

26 J
0
1

-22
0.001

10 6 T

s

0.01

0.1

1

10

Period (s)Figure 1.2 Period-Period derivative (P − Ṗ ) diagram. Lines of constant Bsurf (green, dotted), tauc (red, dashed), and spin-down luminosity (black, long-dashed) are shown. Binary
systems and various neutron star sub-classes, detailed in Sec. 1.1.3, are also indicated.
The distinct magnetic field strength of magnetars is illustrated by their positioning in thisdiagram.

6

Chapter 1. Introduction
1

Duty Cycle (w/P)

0.1

0.01

0.001

0.0001
0.01

0.1

1

10

100

1000

Pulse width (ms)Figure 1.3 Pulsar duty cycles and pulse widths. The vertical striations are the result of thefinite precision of pulse width quantification, not a physical effect. RRATs are marked bysquares to highlight their apparently low duty cycles (e. g. McLaughlin et al. 2006). With
short periods and narrow pulse widths, the upper left island of points is primarily madeup of recycled pulsars. The shaded area marks the region for which P > 10 s, above which
pulsars will have evolved to beyond the death line. Note, additionally, that continuous
sampler level setting on radio telescopes may absorb power from pulses of duration equal
to or greater than the level setting cycle. For the system that discovered the majority ofradio pulsars, the Parkes 20cm multibeam analogue filterbank, this cycle is 1 second.
single pulses or integrated over many star rotations) also comes into play, however will be
addressed in 1.4. The most drastic influence of the interstellar medium is dispersion (whichis also a particularly important aspect of Chapters 4-6). Dispersion will be described indetail, while effects such as scattering and scintillation play a lesser role for the 1 GHz
frequencies used in this thesis, and will be touched upon.
The range of typical pulsar periods is displayed in Figure 1.2. The lower limit on pulsar
periods is closely tied to the equation of state of dense nuclear matter; thus, a pulsar’s
radius and mass (the relationship between which is highly dependent upon the equationof state of the pulsar interior) define its minimum possible spin period. Discoveries of very
fast pulsars challenge models for the equation of state of dense nuclear matter; the fastest
pulsar currently known is J1748–2446ad, spinning 716 times per second with a periodof only 1.39 ms (Hessels 2007). At the other extreme, the pulsar death line predicts a
maximum spin period for observed radio pulsars, however the slowest pulsar, J2144–3933
at 8.5 s, discovered by Young et al. (1999), challenges radio emission models.
We display the pulse width of known pulsars (w, defined at half-maximum intensity)

1.1. Pulsars

7in Fig. 1.3 against the duty cycle (w/P ) of pulsars. It is apparent here that the pulsesof pulsars occupy typically ∼10% of the star’s rotation. The “rotating radio transients”
with small duty cycles on this diagram will be discussed below in §1.1.3.

The intensity, frequency-dependent structure, and observed pulse width of pulsars
are all influenced by a number of effects (dispersion, refractive/diffractive scintillation,
scattering) caused by the cold, ionised plasmas that make up both the interstellar medium(ISM), through which all celestial signals propagate before reaching an observer at Earth.
Any signal of extragalactic origin will also propagate through, and be influenced by, theintergalactic medium (IGM) before passing into our Galaxy.
The existence of the ISM and IGM implies that the space between us and pulseemitting objects is not strictly a vacuum. In fact, the non-unity refractive index, n, of the
ISM causes light propagating through this medium to travel at a group velocity vg = c/n,
slower than the speed of light. As noted by Lorimer & Kramer (2004), the refractive index
of the interstellar medium is strongly dependent upon the plasma frequency of the ISM
and the frequency of the propagating light. The net effect of this is that a radio pulse
emitted by a celestial object experiences dispersion, resulting in an observable frequencydependent delay with time as exhibited in Fig. 1.4. The derivation of the cold plasma
dispersion relation in the ISM is summarised by Lorimer & Kramer (2004), and results ina frequency-dependent lag with respect to a signal of infinite frequency:
∆t∞ =

e2
DM
· 2 ; DM =2πme c f

Z d

ne dl ,

(1.5)

0

f is the frequency of the emission, e is the elementary charge, and me is the mass of anelectron. The dispersion measure, DM, represents the integrated electron column density
along the line of sight from the observer to the emitter (at distance d). Below we use D ≡e2 /(2πme c) ≃ 4.15 × 103 MHz2 pc−1 cm3 s. As implied by equation 1.5, higher frequencies
arrive before lower frequencies, as seen in Fig. 1.4. The DM is a direct observable, given apulse observed to arrive at two frequencies f1 and f2 , at times t1 and t2 , respectively:∆t = t2 − t1 = D · DM · (f2−2 − f1−2 ) .

(1.6)

When creating a frequency-averaged time series for a pulsar, the data must be dedispersed,that is, corrected for the frequency-dependent dispersion given by this equation. It is
implicit in Equation 1.5 that when the DM is measured for an object, one can infer a
distance to a Galactic emitter based on a model for the free electron distribution in the
Galaxy’s ISM. The current standard model used by pulsar astronomers is the “NE2001”

8

Chapter 1. Introduction

Figure 1.4 One pulse from the low-DM pulsar PSR J1456–6843. The lower panel shows aspectrogram, the power in greyscale as a function of frequency and the pulsar’s rotational
phase. The white lines trace DM = 8.6 pc cm−3 . Several effects of the ISM are clear here:
particularly the dispersion of the pulse and the frequency-dependent amplitude modulation(“scintles”) caused by wavefront distortions in the ISM. The upper panel shows the pulse
profile after dedispersion, integrated over the path traced by the white lines.
electron density model of Cordes & Lazio (2002). Pulse dispersion is generally a property
that is beneficial to surveys for pulsing objects; it creates a characteristic signal with whichto distinguish local (esp. man-made) signals from pulses of astrophysical origin. This will
be discussed further in Section 1.4 and is a central element to what makes the propertiesof the discoveries in Chapter 6 so unexpected.
There are two remaining ISM effects which can influence pulsar measurements thatwill play a role later in this thesis: the effects of pulse scattering, and scintillation. These
are mathematically and conceptually developed in detail by both Manchester & Taylor(1977) and Lorimer & Kramer (2004) based on the “thin screen” model of Scheuer (1968)(in which observed ISM effects are caused by a single screen positioned halfway between
observer and emitter). The reader is referred to those texts for details; we report only on
their derived results here.
Essentially, interstellar pulse broadening and scintillation both are caused by the factthat the interstellar medium is both inhomogeneous and turbulent. A propagating wavefront incident upon a scattering screen in the ISM will undergo random phase delays, themagnitude of which are dependent upon the scale, a, of refractive index inhomogeneities(that is, electron density variations, ∆ne ). These delays cause several effects. First, they

1.1. Pulsars

9

will induce a frequency-dependent scatter broadening of a pulse, causing a delta-function
pulse to have an asymmetric tail. Furthermore, interference induced by the wavefront
distortions will cause constructive and destructive interference leading to frequency- andtime-dependent flux variations (scintillation) with a characteristic timescale and bandwidth, depending on the nature of the scattering screen and the relative velocities of thepulsar, Earth, and screen. These effects can have dramatic influences on an observation,
particularly when the characteristic bandwidth of the scintillation is equal to the observing
bandwidth. These effects are typically stronger for low-dispersion measure/nearby pulsars(e. g. Fig. 1.4), and can cause a newly discovered pulsar to not be seen in subsequent
observations if it was discovered near a caustic peak of scintillation.
For the thin screen model considered by the texts noted above, the scattering timescale,τs can be approximated by:τs =e4 ∆n2e 2 −4d f .
4π 2 m2e a(1.7)

Here, τs is in milliseconds, and the observing frequency f is in units of GHz. It is clear thatthe scale and position of inhomogeneities influence the effects of the ISM on the signal. It
is often assumed (and, despite the relation below, holds in observations for some objects)
that the turbulence of the ISM obeys a Kolmogorov energy spectrum (Kolmogorov 1941),
for which τs ∝ f −4.4 . The distance dependence indicates that τs scales with dispersion

measure. In fact, although there is significant scatter, Bhat et al. (2004) give a relationthat is useful when considering the design of pulsar surveys (§1.4.1,§5.2.2):log10 τs = −6.46 + 0.154 log10 (DM) + 1.07 [log10 (DM)]2 − 3.86 log10 (f )(1.8)

This relationship indicates the ability of pulsars observations to assess the applicabilityof turbulence models to the ISM, indicating that on average the frequency dependence of
scattering is actually closer to τs ∝ f −3.86 .

1.1.3

Pulsar Varieties

The vast majority of electromagnetically-emitting neutron stars occupy the island of radio pulsars with intermediate rotational speeds visible in Figure 1.2. However, there areseveral sub-varieties of neutron stars, segregated both by differences in physically understood properties, and by observed (not always yet understood) differences in behaviour. In
radio bands, the behavioural sub-groups most relevant to this thesis are: millisecond pulsars, nulling pulsars, modulated and giant-bursting pulsars, and rotating radio transients.

10

Chapter 1. Introduction

While they will not be discussed, there also exist a number of high-energy emitting neutron star phenomena, some with little or no associated radio emission such as γ-ray pulsars(e. g. Abdo et al. 2010), X-ray dim isolated neutron stars (XDINS Kaplan 2008, Kaplan& van Kerkwijk 2009; and references therein), X-ray binaries, and magnetars (anomalous
X-ray pulsars, soft gamma-ray repeaters Mereghetti 2008).
Millisecond pulsars inhabit the extended lower tail of pulsars with low magnetic fields
seen in Fig. 1.2. Current theories of neutron star evolution label these pulsars with thename “recycled pulsars”, indicating their origin as a pulsar that has undergone a periodof spin-up, powered by the accretion of matter onto the pulsar from a companion star(e. g. Bisnovatyi-Kogan & Komberg 1974, Shibazaki et al. 1989), typically leaving themorbiting a white dwarf or another neutron star. A testimony to this evolutionary track isthat ∼75% of millisecond pulsars are in a binary system. Millisecond pulsars are the moststable rotators of all pulsars, and as such makes them the most promising for detecting
gravitational radiation with the technique of pulsar timing (Verbiest et al. 2008; see §1.2).
Nulling pulsars exhibit periods of time in which the radio emission appears to halt;
depending on the pulsar, these “off” periods last anywhere from one to a few stellar
rotations, anywhere to & 95% of the time the pulsar is observed (Wang et al. 2007).
Nulling phenomena in pulsars are often accompanied by effects such as mode changing—in which the pulse shape oscillates between two distinct profiles—and sub-pulse drift, inwhich components of the pulsar’s radio profile appear to wander in an ordered way between
a range of pulsar longitudes (these effects are well-reviewed by Weltevrede et al. 2006a).
Some pulsars exhibit extreme (apparently intrinsic) variations in pulse-to-pulse flux
density, with some pulses appearing from ten to several thousand times the average flux
density of the pulsar’s single pulses. Some such pulsars exhibit giant pulses (e. g. Staelin &Reifenstein 1968, Johnston et al. 2001, Johnston & Romani 2002, 2003, Knight et al. 2006),
which tend to be of short (µs or less) duration, while others show a smooth pulse energy
distribution with a long tail of bright events (e. g. the extreme emitter of Weltevrede et al.
2006b).
Finally, the rotating radio transients (RRATs) recently discovered by McLaughlin et al.
(2006) appeared upon discovery to represent a new class of sporadically-emitting neutronstar. For RRATs, detectable emission typically occurs for less than one second per day
in the form of single, narrow (a few ms) pulses. The scarce emission from these neutronstars makes them more readily detectable by single-pulse search techniques than the usual
Fourier search techniques used to discovery the majority of pulsars (see §1.4.2). Few

RRATs are known, rendering it difficult to understand the origin of their sparse emission

1.2. Pulsar Timing and Timing Arrays

11(Chapter 4 addresses this topic). Several observed properties appear to make RRATs
distinct from the general pulsar population, including their small duty cycles (Fig. 1.3).
Recent measurements of RRAT periods and period derivatives have led to the conclusions
that Bsurf for RRATs is distinctly higher than average pulsar magnetic fields, and RRAT
periods are statistically longer than those of regularly-emitting pulsars, seeming to further
differentiate the RRATs from the typical pulsar population (McLaughlin et al. 2009).

1.2 Pulsar Timing and Timing Arrays
As noted above, the regularity of pulsars’ rotations make them not unlike precise naturally occurring clocks. This property has yielded the technique of “pulsar timing,” which
involves the measurement and prediction of a pulsar’s pulse arrival times. Pulsar timing employs models for Earth’s motion relative to the pulsar (that is, due to its movement around the Solar System barycentre), for the ISM effects described in 1.1.2, andfor the pulsar itself (e. g. P , Ṗ , binary motion) based on previous observations of thepulsar. The most stable millisecond pulsars have pulse arrival times stable to the orderof < 10−6 seconds over timescales from days to decades (e. g Verbiest et al. 2008). Aftercorrection for modelled effects, any physics or systematics that are unaccounted for will
manifest structure in the pulsar’s timing residuals, which are the actual pulse arrival times
relative to the predicted times of arrival. It is in the post-model-fit timing residuals that
one may search for subtle inaccuracies in the models, or for unmodelled influences on thepulsar, Earth, or the intervening space between.
Greater sensitivity to spatially-correlated effects arises with the concurrent monitoringof multiple pulsars in what is termed a pulsar timing array (PTA: this term was coined by
Romani 1989, although first rigorously employed by Hellings & Downs 1983 to put limits
on gravitational radiation. Foster & Backer 1990 first detailed other physical targets that a
PTA could explore). PTAs aim to detect correlated offsets in the timing residuals of pulsars
spread across the celestial sphere, forming a measurement tool akin to an accelerometer
for Earth within space-time. Targeted effects include errors in Earth’s time standards(currently maintained by atomic clocks), errors in solar system ephemerides (planet masserrors, which would produce strongly correlated residuals along the ecliptic, or unidentified
solar system bodies Champion et al. 2010), and the ultimate target of pulsar astronomy,
gravitational waves (Detweiler 1979).
While there are many efforts in pulsar timing around the globe, there are several
projects and pulsars of particular note. Relevant timing parameters of these projects are
summarised in Table 1.1.

12Chapter 1. Introduction

P140/J0437-4715
J1713+0747
Kaspi
CPSR2
PPTA+
IPTA

Npsr
1
1
2
15
20
37

∼1/C
(weeks)
1-4
3
2
4
1-5
1-4

Data Span(∼years)
10
6.5
7-8
2.5
3-14 (h10.5i)?

Lowestσn (ns)
199
180
200
100
166
20

Npsr
(σn < 1 µs)
–
–
2
6
16
29

ref.
a
b
c
d
e, f
e, g

Table 1.1 A summary of notable historical, and ongoing timing programs. P140 is thelongest-running timing program at Parkes telescope, dedicated primarily to observing PSR
J0437-4715. The Kaspi data set has become a standard in comparing GWB limit/detection
techniques. The CPSR2 data were all taken on the CPSR2 instrument at Parkes telescope.
PPTA is the Parkes Pulsar Timing Array (parameters noted here include additional data as
noted in reference d), and IPTA is the recently-formed International Pulsar Timing Array(a collaboration between the North American Nanohertz Observatory for Gravitational
Waves, the PPTA, and the European Pulsar Timing Array. The parameters quoted here
for the IPTA are optimistic predictions for the IPTA based on optimal performance of theconstituent timing arrays. Note that long-term (.5 yr) timing can lead to increased RMS
residuals due to low-frequency “timing noise” in pulsars’ residuals (e. g. Verbiest et al.
2009); therefore in most cases the shorter-duration projects have generally lower timing
residuals, despite that several projects (e. g. P140, PPTA+) share common pulsars.
Columns: (1) Data ID. (2) Number of pulsars in data set; (3) Typical time between observations (cadence,C); (4) Total data span; (5) Smallest RMS timing residuals over full data span (§1.3.2); (6) Number ofpulsars with RMS residuals better than 1 µs; (7) Publication reference: a, Verbiest et al. (2008); b, Splaver
et al. (2005); c, Kaspi et al. (1994); d, Hotan et al. (2006); e, Hobbs et al. (2009); f, Verbiest et al. (2009);
g, Hobbs et al. (2010), Jenet et al. (2009), Ferdman et al. (2010).

1.3. Gravitational Wave Astronomy

13

1.3 Gravitational Wave Astronomy
Gravitational radiation is a direct prediction of Einstein’s space-time metric equations ofgeneral relativity: a gravitational wave (GW) is a propagating quadrupolar fluctuation in
the curvature of space-time that is produced when a mass is accelerated, mathematicallyanalogous to the electromagnetic waves produced by an accelerated electron. No gravitational waves have yet been detected, however the first indirect observational evidence for
GWs arose from the timing of the binary neutron star PSR B1913+16 (Hulse & Taylor1975) by (Taylor & Weisberg 1982), who showed that the system is losing orbital energy
at precisely the rate expected from the energy lost from the system due to GW emission(a discovery which resulted in the Nobel Prize for physics!).
As a gravitational wave (GW) propagates, it creates small distortions in space-time
perpendicular to its propagation axis. Provided the wave is within the pulsar timing
band, such a wave is theoretically directly detectable by PTAs through its quadrupolar
fingerprint left as a correlated signal in the array. Given many sources of unresolved
GWs, they will produce a stochastic GW background (GWB) that is detectable in PTA
data through its correlated signature in timing residuals (Hellings & Downs 1983). Givena well-timed, individual pulsar, limits may be placed on the presence and strength ofgravitational radiation or a GWB, however a definitive detection cannot be separated
from other effects.

1.3.1

Pulsar-detectable Gravitational Wave Sources

Pulsar timing is sensitive to unpredicted changes in the Earth’s movement on timescales
between the length of sustained pulsar observations (some timing data sets now reachup to several decades), and the cadence of pulsar observations, at most typically one
every two weeks. Consequently, pulsar timing data have sensitivity to the low-frequency(nHz . fgw . µHz) gravitational wave spectrum, complementary to gravitational wave
detectors such as the Laser Interferometer Gravitational Wave Observatory (“LIGO”) andthe planned Laser Interferometer Space Antenna (“LISA”). The gravitational radiation
spectrum, various theoretical sources and limits of current and future GW detectors are
summarised in Fig. 1.5.
Theoretical sources of gravitational wave emission in the pulsar timing band include
GWBs from primordial intermediate-mass black holes (Saito & Yokoyama 2009), cosmic
strings (Caldwell et al. 1996, Damour & Vilenkin 2005), and tensor perturbations from therapid expansion of the inflation epoch in the early Universe (Maggiore 2000, Grishchuk

14

Chapter 1. Introduction

Figure 1.5 The gravitational radiation spectrum for various GW sources in the frequency
ranges in which pulsar, LISA, and LIGO are sensitive. The three detectors are complementary in the spectrum, and as such are sensitive to different GW sources. In the pulsar
band, we show the expected signal from individual SMBH binaries (shown at the distanceof the Virgo cluster of galaxies), and three predictions for the GWB from supermassive
binaries; the upper and lower dotted lines show the maximum allowed GWB predictedby Wyithe & Loeb (2003) and the semi-analytical prediction of Jaffe & Backer (2003),
respectively, while the hatched region shows the predictions of Sesana et al. (2008), incorporating a range of uncertainties in galaxy formation parameters and exhibiting a spectral
turnover caused by the paucity of supermassive systems emitting at higher frequencies.
This figure was adapted from that presented in Yardley et al. (2010).

1.3. Gravitational Wave Astronomy

15

2005, Boyle & Buonanno 2008).
The strongest expected sources of gravitational radiation for pulsar timing come fromthe most massive compact objects in the Universe: supermassive black holes (mass >106 M⊙ ,hereafter SMBHs), which reside at the center of nearly all massive galaxies. SMBHs willproduce the strongest gravitational waves in the Universe when in a closely bound binary
system with another SMBH, emitting gravitational waves with a period half of the system’s orbital period. Therefore, PTAs have sensitivity to these systems when the orbital
period of the binary is within the time scale of the pulsar timing experiment: that is, when
the orbital separation is r ≪ 10 pc (for more details see §2.3). The expected number ofindividual binary systems detectable by pulsar timing arrays was explored by Sesana et al.
(2009). In a Universe built up by the hierarchical merging of galaxies, a stochastic GWB
from many SMBH binary systems is also expected, with a strength and spectral form dependent upon the mass, number density, and cosmological distribution of binary systems
in the Universe (which, in turn, have a strong dependence on the parameters of galaxy and
SMBH growth, such as the time-dependent merger rate of galaxies, the post-merger inspiral timescale for SMBHs, and the contribution of accretion versus mergers to SMBH mass
growth). This implies a close link between pulsar timing and galaxy evolution, throughwhich limits from pulsar timing on the astrophysical gravitational wave background canbe used to constrain models of galaxy formation. Until a background is detected, pulsar
timing studies rely on galaxy evolution models to give predictions for the amplitude andspectral form of the GW signals; however with very strong limits on or a detection of
the GWB, pulsar timing will be able to constrain various facets of galaxy evolution thatare otherwise difficult to derive through electromagnetic observation. The predicted formof the astrophysical GWB has been explored in basic hierarchical-merging contexts by
Rajagopal & Romani (1995), Jaffe & Backer (2003), Wyithe & Loeb (2003), Enoki et al.
(2004), and expanded for the inclusion of uncertainties in several galaxy formation parameters by Sesana et al. (2008). However, a major—and potentially detrimental—uncertainty
in these models remains in the observationally unconstrained inspiral timescale for SMBH
binaries after a galaxy merger, which the predictive models generally neglect. It is currently not known whether there is sufficient fuel for orbital energy dissipation during
SMBH binary inspiral for the SMBHs to coalesce within a Hubble time, giving rise to thepossibility of “stalled” SMBH binary systems that remain at orbital separations beyond
those detectable by pulsar timing. This is colloquially termed the “last parsec problem”,
referring to the ineffectiveness of energy dissipation mechanisms to shrink a binary below
separations of ∼1 pc (Begelman et al. 1980, Merritt & Milosavljević 2005).

16

Chapter 1. Introduction

1.3.2

Gravitational Wave Signals and Timing Sensitivity

Here we review the mathematical representation of the signal from gravitational wave
backgrounds and individual SMBH binary sources, and the sensitivity of pulsar timing
to GW emission. The fractional contribution of the GWB to the energy density of the
Universe in a logarithmic frequency interval can be expressed as:Ωgw (f ) =hc (f )2
2π 2 3
f
S
(f
)
;
S
(f
)
=
,
h
h
f
3H02

(1.9)

where Sh (f ) represents the total power spectral density of gravitational waves at frequency
f (using the “one-sided” convention of Jaffe & Backer 2003), and hc (f ) represents thecharacteristic strain spectrum of the GWB. For most predictions of gravitational wave
background, the characteristic strain follows a power law with frequency, ashc (f ) = A



f
f0

α

,

(1.10)

with amplitude A, and spectral index α at a normalising frequency f0 = 1 yr−1 . Most
backgrounds can be described in this way; e. g for primordial GWs, α ranges −0.8 to −1.0

with amplitudes 10−17 < A < 10−15 , while for the GWB from SMBH binaries, predictions

typically find that α = −2/3 and 10−16 < A < 10−14 .

The astrophysical background of supermassive binaries, as noted above, is defined by

the density and distribution of SMBHs in the Universe. This is well-illustrated by thesimple formulation for the characteristic strain spectrum of SMBH binaries given by Jaffe& Backer (2003):
2

hc (f ) = f N (z, f, m1 , m2 )Z

h2s (z, f, m1 , m2 ) dz dm1 dm2 .

(1.11)

Here, z is the cosmological redshift of the source (analogous to distance), m1 and m2 are
the masses of the constituent black holes, N (z, f, m1 , m2 ) represents the number densityof systems per f, m1 , m2 , and redshift bin, and hs is the strain contributed by one binary
system:r

hs = 42 (GM)5/3· (2π/P )2/3
5 c4 D(1.12)(Peters & Mathews 1963, Thorne 1989), where M = (m1 m2 M −1/3 )3/5 is the chirp massof a binary system, and D is the distance to the object. Note how the strain scales with
distance at hs ∝ D −1 (that is, it is not analagous to e. g. a flux density).

Given the steep spectrum predicted for all GWBs in the pulsar timing band, the1.4. Pulsar Searches and Fast-Transient Radio Astronomy

17

most stringent limits set by pulsar timing on GWBs are at the lowest frequency—that is,timing arrays are most sensitive to these backgrounds at frequencies 1/T , where T is thetime span of the arrays’ observations. In fact, pulsar timing’s sensitivity to gravitational
waves depends on a number of factors, also including the accuracy of a pulsar’s timing
model over the span of the experiment (timing precision is quantified by the root-meansquared of the timing residuals, σn ), the cadence and regularity of observations, and forgravitational wave detection with pulsar timing arrays, the sensitivity depends on thenumber of pulsars in the array and their distribution on the sky. The sensitivity togravitational wave backgrounds of α = −2/3 was derived by Jenet et al. (2005) for pulsar

timing arrays; they find that an array of Npsr pulsars can detect a gravitational wave
pbackground to a significance of S ∝ (Npsr (Npsr − 1)) (in noise-dominated data as wepcurrently have, however S ∝ Npsr when S becomes significant). They furthermore derive
the dependence of the lowest detectable A on PTA observing parameters (or, for the caseof single pulsars, the dependence of the limit placed on A), finding A ∝ σn T −5/3 N −1/2 ,where N is the number of observations for each pulsar in the array. We can see here thatdata sets of long duration provide significant improvements in pulsar timing’s sensitivity
to the supermassive binary GWB, and further critical improvements come with a better
understanding of long-term intrinsic pulsar instabilities and observing hardware/softwaresystematics; both of these contribute to a larger σn of a pulsar’s timing residuals.
Because of data-fitting procedures performed during timing, the sensitivity of pulsar
timing arrays to single sources of gravitational radiation is not straight forward to define.
However, it has been formally calculated for pulsars in the Parkes Pulsar Timing Arrayby Yardley et al. (2010), whose single-source sensitivity curve is shown in Fig. 1.5 as the“PSR single source limit”.

1.4 Pulsar Searches and Fast-Transient Radio Astronomy
1.4.1

Considerations in Pulsar Survey Design

As one can infer by pulsar’s fast periods, short impulse durations, and the need to correct
for a pulse’s dispersion (§1.1.2), the systems employed for observing pulsars are highly
specialised.
An ideal pulsar search system will sample the sky at intervals of tsamp . 1 ms inindividual frequency channels of width b over a total bandwidth B. Blind surveys mustsearch a range of DMs, and aim to minimize b and tsamp such that the observed width ofa pulse with intrinsic width wint is usually dominated by τs (Eq.s 1.7, 1.8), rather than

18

Chapter 1. Introduction

instrumental pulse-broadening effects. The effective pulse width is given byq
2 + ∆t222
weff = t2samp + wint
Dsm + ∆tDerr + τs ,(1.13)

where ∆tDsm represents the inter-channel smearing of a pulse due to its dispersion, quantified by ∆tDsm = 8300 DM b/f 3 seconds, while ∆tDerr is the broadening of a pulse due to anerror of magnitude ∆DM from the source’s true DM, given by ∆tDerr = 8300 ∆DM B/f 3
seconds (where b, B, and f , the centre observing frequency, are in units of MHz). These
instrumentation requirements are realised by filterbanks, which can provide the necessary
sampling to preserve the condition ∆t2Dsm + ∆t2Derr < τs2 . However, b and tsamp may
not be made infinitesimally small due to limits of data transfer, processing, and storagep
speeds, and the necessary limitation that B · tsamp > 1. Sampling npol polarisations

in each of Nch = B/b channels at nbits digitisation levels, data is produced at a rateof npol nbits Nch /tsamp . Current algorithms and computing hardware cannot maintain asearch of significant dispersion measure space in real time without considerable expense,
so data typically must be stored for later offline processing. As such, Nch and tsamp arelimited by the rate that data may be archived, and surveys must carefully consider theirtarget sources, and trade-offs between the various observing system parameters.
Nevertheless, a search’s sensitivity to a pulsar whose peak flux is Speak can be determined based on the radiometer equation:
Speak =

mT β
p sys
,
G npol tint B(1.14)

where Tsys is the “system temperature” of the telescope in Kelvin, G is the telescope’s gain
in K Jy−1 , m is the signal-to-noise ratio of a detection (with a minimum threshold usually
set for blind pulsar searches), and tint is the total integration time of the observation. β
is a factor included to represent signal losses due to system imperfections e. g. one-bit
digitisation or non-rectangular channel bandpasses (e. g. Manchester et al. 2001, Lorimer& Kramer 2004), and is typically .1.25.

1.4.2

Periodicity Searches and the Re-advent of Single Pulse Searching

The first pulsar was discovered using a chart recorder by Jocelyn Bell, marked by its
bright, periodic single pulses. In fact, the first ∼50 pulsars were discovered by searchingfor their single-pulse emission. However as computing power advanced, the developmentof filterbanks and digital signal processing allowed the use of search techniques that use
Fourier Transforms and/or the averaging (“folding”) of data over a range of periods to1.4. Pulsar Searches and Fast-Transient Radio Astronomy
Periodicity
searchFFA
FFTChannelised(e.g. filterbank)
Data

Single pulse
searchDispersion
trial

Cand. list
(P, DM)

Harmonic
sums

Event
searchPre-set thresholds

Cand. List
(time, filter, DM)

Apply filter

Filtertrial? NY

19YDetermine
best cands
N (max DM, P)

Another
DM trial?
Y

Diagnostic
plots

Manualinspection

N

(a)

(b)

Figure 1.6 (a) Here we show the basic flow of periodicity and single pulse searches for broadband pulsar and transient phenomena. The periodicity search is described in §1.4.2. Bothsearch modes share the most computationally heavy aspect of searches—dedispersion—and benefit therefore in computation time and cost if performed contemporaneously. The
dark grey panel lays out the primary components of the single pulse search methods
outlined in Cordes & McLaughlin (2003): dedispersion, thresholding (setting a limit on
candidate numbers based on the local interference environment), matched filtering, anddiagnostic plotting. As noted in their paper, the choice of matched filter shape should
be designed to match an expected pulse shape, for instance the exponential tail a deltapulse may acquire after propagation through the interstellar medium; however, in practicea progressive “smoothing” (i. e. the boxcar filter used in Chapters 4 and 5) is generally
sufficient. Below, we reproduce an example single-pulse diagnostic plot from (Cordes &McLaughlin 2003). In their method, single pulse candidates are assessed across the entiretyof the pointing (resulting here in a clear detection of pulses from the millisecond pulsar
B1937+21).

20

Chapter 1. Introduction

allow increased sensitivity to the periodic signals exhibited by pulsars. Today, a typicalpulsar search proceeds as outlined in the left panel of Figure 1.6, beginning with thededispersion of multi-channel data into a band-integrated time series for each DM trial(with trial step intervals usually chosen to maintain ∆tDerr . 2∆tDsm for pulsars with a
DM between two steps). The time series are then used to perform periodicity searches(e. g. using the Fast Fourier Transform, Brigham 1974, Ransom et al. 2002, or a fast
folding algorithm, Staelin 1969), performing harmonic summing of Fourier components toincrease sensitivity to pulsars of small duty cycle (Ransom et al. 2002). Such techniques
have allowed the discovery of great numbers of pulsars, many that would have been too
weak to discover by their single pulses alone.
However, given the specifications of the pulsar search systems outlined above, it is
not surprising that pulsar data and observing systems are ideally suited for performing
searches for single-event, short-duration pulses from not only neutron stars, but othermiriad other possible celestial sources. Single celestial impulses will, too, be dispersed,
requiring fast sampling and narrow sub-bands with which to correct for dispersion effects.
As such, past pulsar surveys represent one of the few historical records of the sky capable ofdetecting transient radio events of duration ≪1 second, while new surveys present further
search opportunities, and (given sufficient computing power) the chance to find events inreal-time, allowing prompt follow-up observations.
In the past decade astronomers have begun to recognise this fact, returning to archival
pulsar surveys that have already been rigorously searched for periodic signals, and applying
new algorithms to search for transient, one-off pulses that would have been missed by
Fourier search techniques; Fourier transform searches are insensitive to single bursts ofemission, and become less sensitive to some periodic sources due to to erratic or sparseemission, short duty cycles, and long rotation periods (the latter due to low-frequency
noise in the data, e. g. Deneva et al. 2009). While the first blind searches for single pulses
began a number of years prior to its publication, Cordes & McLaughlin (2003) first laidout the known sources of transient radio emission, considering a “transient phase space”
that exhibits these phenomena (Fig. 1.7), and developing a set of standard methodologies
for single pulse searching (see section §4.1 for a more complete review of blind single pulse
searches). The primary features in the Cordes & McLaughlin search methods are outlined
and discussed in Figure 1.6.
It is now recognised that there may be many transient phenomena detectable in pulsar
surveys that, if detected, may provide insight into a variety of emission sources. These arereviewed below.

1.4. Pulsar Searches and Fast-Transient Radio Astronomy

TB=1040K

1020 K

1030 K
Lorimer
Burst

HE

OH Masers

RRATs

RE

0

B0656+14NT

Crab
B0540-69
nano-pulses giant pulses

HE

5

AG
N

Crab
giant pulses

COType II, III
Solar Bursts-5

AD LeoUV Ceti

Jupiter-10

CO

PulsarsIN

log [ SpeakD2 (Jy kpc2) ]

RE

NT

10

21

-5

1010 K
0

Brown Dwarf
LP944-20

100 K

5

10

log [ f wint (GHz s) ]Figure 1.7 Here we show an augmented display of the transient radio phase space of Cordes& McLaughlin (2003). It reflects the known sources of transient radio emission, most ofwhich are detectable in pulsar data streams given sufficient tsamp , b, and flux sensitivity.
These are detailed in §1.4.3. Overlaid are contours of constant brightness temperature,
TB ; recall that at low frequency (suitable for radio wavelengths), TB can be represented
by the Rayleigh-Jeans approximation to the Planck law, allowing the derived expression
TB ≃ Speak λ/(2 kB Ω) = Speak D 2 /[2 kB (f wint )2 ], where λ is the observing wavelength, kB
is Boltzmann’s constant, Ω is the solid angle of emission, D is the distance to the source,
and f is the observing frequency. TB ≃ 1011−12 K represents the maximum brightness
temperature of an incoherent synchrotron emission source; above this limit, Compton selfscattering prevents incoherent processes(Readhead 1994, Singal 2009). The large range inluminosity space for the Lorimer Burst reflects the unknown distance to the object (see§1.4.4).

22

Chapter 1. Introduction

1.4.3

Known Sources of Fast Transient Radio Emission

It is clear in Figure 1.7 that there are only a handful of known classes of radio transients.
We note that above TB ∼ 1015 K, this phase space has until recently been restricted toperiodicity-focussed searches using Fourier techniques.

Pulsars form a fairly compact island on this diagram. Several neutron-star related
objects, all of which have distinctive single-pulse emission, have isolated positions; forinstance, pulsars such as the Crab, which in its integrated emission lies within the pulsar
island, emits giant (105 Jy) single pulses as well as high-intensity bursts of nanosecond
duration. High pulse-to-pulse modulation and periodic nulling represent other types ofsignal variance observed in the single pulse emission of some pulsars (e.g. Herfindal &Rankin 2007, Kramer et al. 2006). Such neutron star-related phenomena promise insights
into the localisation and power source of radio pulsar emission. Single pulse searches
have the ability to discover extreme examples of these sources, and may serve to reveal abridging population or define a distinction between steadily-emitting pulsars and pulsars
with excessive nulls. This could help reveal whether the rotating radio transients of §1.1.3

are (in physical mechanism) just extreme examples of nulling pulsars (see Chap. 4) or
distant, modulated pulsars, such as radio pulsar B0656+14, which has a lognormal pulse
energy distribution with a high-energy tail that sets it apart from typical pulse energy
distributions. This pulsar was found to have a single-pulse energy distribution with a longhigh-energy tail, leading to the supposition of Weltevrede et al. (2006b) that the pulsar
is a source which, if it were more distant from Earth (thus making its average emission
undetectable), might be observed as an RRAT.
Non-neutron star transient phenomena that exist in the transient sky, however were notdiscovered through pulsar-data searches, include active galactic nuclei, which exhibit
intrinsic intermediate-duration flares (e. g. Aller et al. 1985, Kawai et al. 1991, Robson et al.
1993) as well as intra-day variability caused by interstellar scintillation (see discussion
on AGN variability in §3.3). Flares have been observed to arise on second- to hourlong timescales from a number of sources: for instance brown dwarfs, giving us insight
into the magnetic activity in these stars (Berger et al. 2001), active stars (e. g. UVCeti, AD Leo, and several others Jackson et al. 1989)—revealing cycles possibly similar
to those of the sun—and even Jupiter (Goldreich & Lynden-Bell 1969, Aubier et al.
2000), which emits decametric radiation interpreted to arise from the volcanically active
Io causing currents to flow between Jupiter and its moon. Such signals, if detected inother star systems, could indicate the presence of extra-solar planets (Willes & Wu 2005).
It has long been recognised that the sun is very active in short-duration radio signals,

1.4. Pulsar Searches and Fast-Transient Radio Astronomy

23

emitting ∼MHz-wavelength, swept-frequency solar bursts classified into several spectral
types. These typically arise from non-thermal emission formed in interactions of solar
flares with the stellar wind or with the solar magnetosphere Loughhead et al. (1957a),
Poquerusse et al. (1988), Mann et al. (1996).

1.4.4

Unknown & Theoretical Sources of Fast Transient Radio Emission

Theoretical arguments predict fast transients from a variety of non-neutron star origins,such as coalescing systems of relativistic massive objects (Li & Paczyński 1998,
Hansen & Lyutikov 2001), evaporating primordial black holes (Rees 1977), or supernova events (e.g. Colgate & Noerdlinger 1971). If such bursts could be routinely
detected they would provide a powerful cosmological probe. Because the intergalactic
medium harbours the vast majority of baryonic matter in the Universe, and we expect ahigh ionisation fraction of this matter, measurements of DM as a function of redshift would
provide a descriptor for the density of the Universe over a range of cosmological redshifts,
elucidating the baryonic content and ionization history of the Universe. In addition, if
bursts associated with relativistic-object coalescence were discovered, an electromagnetic
indicator of the gravitational coalescence of such objects would provide the direction andevent time of a gravitational wave signal; this would lower the detection threshold ofdetectors such as LIGO manyfold.
Given this, perhaps the most exciting recent discovery from single pulse searches arose
coincidentally to the author’s initiation of doctoral research. The discovery was a one-off
burst tentatively identified as having an extragalactic origin (Lorimer et al. 2007), thathas come to be known in the community as the Lorimer Burst. This discovery wasa ∼30 Jy pulse, which appeared to show frequency-dependent pulse scattering consistent

with that predicted by a Kolmogorov spectrum (with τs ∝ f 4.8±0.4 ; recall §1.1.2) andshowed a strongly swept-frequency signal consistent with cold plasma dispersion, at adispersion measure of 375 pc cm−3 . The consistency with the signatures expected frompulsed astrophysical phenomena led to the suggestion that this was the first detectedradio impulse of extragalactic, cosmological origin; based on its large dispersion measure,the pulse appeared to originate from a redshift of z ∼ 0.3.

The discovery of this pulse was the result of reprocessing an archival pulsar surveyof the Magellanic Clouds, at the time representing one of the few data sets of significant
volume ever searched for single pulses. The fact that the search uncovered such a “goldstandard” event, ∼100 times the detection threshold of the data, seemed to imply thatmany more should be detected in subsequent searches. At the inception of this thesis,

24

Chapter 1. Introduction

the origin of the Lorimer Burst remained ambiguous, with no similar detections since theoriginal discovery (Deneva et al. 2009, Keane et al. 2010). However, we provide a curious
advancement on this story in Chapter 6.

1.5 Thesis Outline
The aim of this chapter has been to provide a backdrop to the diversity of topics within
Pulsar Astronomy. We have outlined the extreme birth, lifetime, and physical propertiesof pulsars, and have revealed how pulsars are being used to study a vast range of astrophysical topics. We have shown that the prediction of pulsar arrival times, and the useof many pulsars in a “pulsar timing array,” can reveal insights into the study of binary
supermassive black hole systems through pulsars’ potential to detect gravitational radiation. Additionally, we have stressed the important role that specialised pulsar observing
systems have in making contributions to fast-transient radio astronomy: in particular, howsearches for single short-duration radio events in pulsar data can reveal insights into pulsar
emission through the discovery of sparsely-emitting neutron stars, provide valuable probesof the evolution of the Universe and explosive extragalactic phenomena, and discover yet
unknown celestial radio emitters.
In Chapters 2 and 3, we explore the binary supermassive black hole population. Chapter 2 examines whether currently recognised electromagnetic tracers of SMBH binaries will
be able to discover sources emitting gravitational waves in the pulsar timing band, anddiscusses the prospects of finding such a system. Chapter 3 then reports on an extensive
survey for SMBH binaries in archival radio data, developing further one electromagnetic
identification technique discussed in Chapter 2. The results of this search are interpreted
in the context of the “last parsec problem” (§1.3.1) and the measurment of the inspiralefficiency of SMBHs after a merger.
Chapter 4, we introduces new methods of single pulse searching and their application
to search for transient events in archival pulsar data at high galactic latitudes. This search
resulted in the discovery of 14 new neutron stars, 12 of which are sparse emitters (e. g.
RRATs or extreme nulling pulsars), and one of which, J0941–39, appears to represent a
cross-over between a nulling pulsar and an RRAT. We use the new discoveries to explore
the origin of RRAT emission, and compare RRAT properties to those of normal pulsars.
Chapter 5 presents the search of the ongoing High Time Resolution Universe Pulsar
and Transients Survey (HTRU). This survey covers a sky area which overlaps with previous
pulsar surveys, however the new hardware systems used in HTRU afford a greatly increased
sensitivity to narrow single impulses. In this chapter present software which employs the1.5. Thesis Outline

25

search methods of Chap. 4, and explain the integration of this transient-searching software
into the traditional Fourier-based search used in the survey. We quantify the survey’s
sensitivity to single pulses, and discuss the 12 new discoveries in comparison to previous
surveys of the same region and in the context of the dynamic radio sky.
Chapter 6 introduces the discovery of Perytons, a terrestrial, swept-frequency phenomenon with remarkable similarity in their frequency sweep-rate to the cold plasma dispersion relation. The delay distribution of these objects peaks at an equivalent dispersion
measure of ∼380 pc cm−3 , drawing significant doubts on the extragalactic interpretationof the Lorimer Burst’s origin (§1.4.4). We suggest a possible origin for the Perytons, anddiscuss the consequences for future radio transient searches.
Finally, Chapter 7 reviews the results and conclusions made in this thesis. We also
include a brief discussion of key future projects that would build on the advances of thework presented here.

2
The Co-detectability of Electromagnetic andGravitational Wave Emission
from Binary Supermassive Black HolesThe world thus appears as a complicated tissue of events, in whichconnections of different kinds alternate or overlap or combine andthereby determine the texture of the whole.
—W. K. Heisenberg
In the next two chapters, we explore the link between pulsar timing and SMBH evolution by considering several methods that can identify small-orbit supermassive binary
black holes. Directly detecting SMBH binaries through their electromagnetic signatures
at a range of orbital separations will make important contributions to the prediction of thestrength and detectability of SMBH-origin gravitational waves, enabling the determination of the post-merger SMBH inspiral efficiency and exploring the prominence of “stalledbinaries.” That relationship is explored in Chapter 3.
In this chapter, we lay a quantitative outline for several binary identification methods,
including pulsar timing and two techniques of supermassive binary identification throughelectromagnetic emission from regions closely associated with SMBHs. The discovery ofa system through its electromagnetic emission that is in the detectable frequency rangeof timing arrays would elucidate a definite target for pulsar timing arrays, and lead to agreat number of scientific returns in the system’s study, reviewed below. We determine themass, distance, and orbital separation of binary black holes which may be co-detectableby both electromagnetic and gravitational wave identifiers, and touch on the prospects offinding such a system in a targeted survey.
27

28

Chapter 2. Co-detectability of Gravitational and Electromagnetic Waves

2.1 Introduction
Binary supermassive black holes play an important role in a number of areas in astrophysics, including the formation of galaxies, galactic dynamics, and, as outlined in §1.3.1,

current and forthcoming gravitational wave science. Although few binary supermassive
systems are known, given that merging galaxies are observed in the local Universe, asdetailed below, it follows that numerous binary SMBHs must exist in the local Universe.
While pulsar timing progresses down to the sensitivity limit to detect these objects throughtheir gravitational emission, the electromagnetic identification and observation of such
systems has thus far been sparse, due primarily to the lack of identified observationalsignatures of the systems.
However, at present identifying electromagnetic signatures of SMBH binaries has significant merits, especially if a system can be identified which should be emitting gravitationalradiation within the sensitivity limits of pulsar timing. For such cases, the knowledge ofthe sky position and a rough orbital solution of the source will raise the sensitivity of a
PTA to the object manyfold (e. g. Jenet et al. 2004). Binary systems are also intrinsic
cosmological distance tools; given an orbit known through either the long-term presenceof a gravitational wave signal in timing data, or through constant electromagnetic monitoring, a spatially resolved binary is a precise standard ruler. Furthermore, the knowledgeof a host galaxy for a gravitational wave signal through electromagnetic identifiers allows
one to break the degeneracy between the source distance (measurable by the GW signal,
Corbin & Cornish 2010) and the cosmological redshift, making the source a standard candle (or, as it has been called for the GW case, a “standard siren”, Schutz 1986). Finally,
the measurement of the differences in effects of the GW signal on a pulsar and on Earth(commonly termed the “pulsar” and “Earth terms” of a signal) can afford clues into thehistory of the binary’s inspiral, and a comparison with the current state of AGN emissivity
for the SMBH binary identification techniques discussed below. The electromagnetic identification of current SMBH binaries in galaxies will aid in the understanding of the natureof binary hosts: e. g. galaxy morphology, large-scale radio emission features, galaxy type,dynamical state, etc., can lead to more precise estimates of binary merger rates, and would
allow measurements of the influence of accretion versus mergers in SMBH growth. Thesemerger rates can feed back to providing direct predictions of the expected gravitational
wave background signals for pulsar timing.
Below, we explore the overlap between various currently recognized techniques forthe detection of binary supermassive black hole systems, and determine whether there
is a set of binary sources that can be co-detected through their observable gravitational

2.2. Electromagnetic SMBH Signatures

29

Figure 2.1 A broad sketch of the birth and evolution of a supermassive black hole binary.
Some physical processes are ongoing, to some degree, throughout the entire evolutionary
process (e. g. dynamical friction, emission of gravitational waves), however this sketch
accents the sequence of significant events and dominance of various energy dissipation
mechanisms contributing to the binary’s inspiral. Image credits: Panel 1, S. Kazantzidis(KICP/U. Chicago); Panel 2, De Propris et al. (2007); Panel 5, C. Henze (NASA)and electromagnetic signatures. The sensitivity of pulsar timing arrays to gravitational
waves from single supermassive black hole binary systems was outlined earlier (see §1.3.2).

We outline hypothesised electromagnetic signatures of SMBH binaries in §2.2 below, andthe physical parameters of systems in §2.3. We then consider the systems identifiable

using two currently recognized methods of small-orbit binary black hole detection throughelectromagnetic radiation: 1) spatially resolving a binary system’s active galactic nuclei(§2.4.1), and 2) spectrally resolving offset or double-peaked broad- or narrow-line regions
from a binary active galactic nuclei (§2.4.2). In §2.5 we investigate what systems could beco-detectable by these electromagnetic identification techniques and pulsar timing arrays,and discuss the prospects of actually finding a system which is simultaneously identifiableby both its GW emission and one or both of these electromagnetic identifiers in §2.6.

2.2 Electromagnetic Signatures of SMBH Binaries
Details of the formation and evolution of supermassive black hole binaries are reviewed indetail in the next chapter, however to initiate the reader, a broad sketch of supermassive
black hole binary evolution is laid out in Fig. 2.1. While very few post-merger binary
SMBH systems are known, observationally there are several ways to identify galaxies
with binary nuclei, which are reviewed briefly below (see also Komossa 2006 for a good
overview).
Pairs of galaxies/quasars with small angular separations and similar redshifts, as well
as galaxies which show disturbed structure and tidal tails in the optical and infrared, canindicate interaction between widely separated stellar cores containing SMBHs in the earlystages of host galaxy merger (e.g. Myers et al. 2008, De Propris et al. 2007, Wen et al.

30

Chapter 2. Co-detectability of Gravitational and Electromagnetic Waves

2009). However, after sufficient relaxation of the host galaxy, pairs are difficult to identify
through electromagnetic signatures without a direct tracer (for instance, distinct emission
from an active galactic nucleus). Nevertheless, secondary signals that have been modelled
as a consequence of a close or bound binary SMBH include quasi-periodic oscillationsin flux density (OJ287, Valtonen et al. 1988), abnormal jet morphology such as helices
or X-shaped radio jets (Saripalli & Subrahmanyan 2009, Lal & Rao 2007), and spatial
oscillations of an active galactic nucleus (3C66B, Sudou et al. 2003). For the last of these,
Sudou et al. proposed orbital parameters in the range of a system we would endeavour todiscover. However, in this instance the Sudou et al. interpretation is also a good exampleof the danger in using indirect signatures to search for binary black holes. Soon after the
Sudou et al. (2003) publication, Jenet et al. (2004) noted that the system could not bea binary with the proposed orbital parameters; if it were, it should be emitting a strong
GW signal, which was distinctly absent in pulsar timing data. We reference 3C66B below
as a proxy for a system of interest.
A binary identification technique that has arisen in recent years aims to identify sources
with abnormal (i. e. double or offset) broad/narrow emission line regions, implying active
galactic nuclei (AGN) moving at high relative velocities to each other or to the hostgalaxy (e. g. Peterson et al. 1987, Comerford et al. 2009, Boroson & Lauer 2009, Decarli
et al. 2010). In some of these cases, other explanations have better suited the observed
characteristics of each source (e. g. Boroson & Lauer 2009, Gaskell 2010, Chornock et al.
2010), however for some systems most other explanations can be ruled out, and estimatesof the binary system parameters support the existence of a sub-kiloparsec, supermassive
binary system.
Most promising are systems such as 3C75, which shows two jet-emitting radio AGN
cores, NGC 6240 (Komossa et al. 2003), and other spatially resolved double X-ray or radio
emitting AGN (e.g. Wang & Gao 2010), which are straightforward to identify and canprovide a direct detection of a double SMBH system. The core regions of radio-luminous
AGN have been observed to show a distinct radio continuum spectrum which typicallypeaks at radio frequencies above ∼1 GHz, giving core components a characteristically flat
two-point spectral index (α > −0.5, where radio flux Fν ∝ ν α ) at GHz frequencies (e.g.

Slee et al. 1994). This feature distinguishes the region most directly associated withthe host black hole from outer features such as larger scale jets or hotspots, which show
power-law spectra generally at or steeper than α = −0.7. Taking advantage of these uniquespectral signatures of the core regions of radio-emitting AGN at high radio frequency (1to 90 GHz), it is possible to spatially resolve and identify AGN containing multiple cores2.3. Physical Preliminaries

31

using multifrequency gigahertz spectral imaging. It is this property that has provided the
supporting evidence for the existence of the smallest-separation binary supermassive blackhole currently known, 4C +37.11 (0402+379; Rodriguez et al. 2006).

2.3 Physical Preliminaries of SMBH Binary Systems
We use the following conventions in notation: two black holes have masses m1 , m2 whereq ≡ m2 /m1 < 1. The reduced mass, µ, of a system is µ = m1 m2 /M , where M = m1 + m2 .

Variables of the format Qi ph refer to the value Q of ph (ph = bin for the binary orbital

quantity, and ph = gw for the quantity of the related gravitational waves) measured by i(where i = o is observed quantity at Earth, and i = s the physical quantity in the source
rest frame). For example, a binary’s source-rest-frame orbital period is Ps bin .
A binary black hole at redshift z in a Keplerian orbit will have a period observed at
Earth of Po bin = Ps bin · (1 + z), and an orbital semi-major axis of:
a=



√

Po bin GM 2/3.
2π(1 + z)(2.1)

If the redshift z of a SMBH binary’s host is known, we can estimate the observed angularseparation of two discrete objects by calculating the co-moving radial distance to theobject:cD(z) =
H0

Z z
0

dz ′
,
E(z ′ )

(2.2)

where E(z)2 = Ωm (1 + z)3 + (1 − Ωm − ΩΛ )(1 + z)2 + ΩΛ , (e. g. Peebles 1993). Here,
we assume a geometrically flat Friedmann-Robertson-Walker Universe, with Ωm = 0.27,ΩΛ = 0.73, and the Hubble constant H0 = 72 km s−1 Mpc−1 . The angular diameter
distance to an object provides the observed linear-to-angular scaling at the target object:
dA =

ℓ
D(z)
=
,
θ
1+z

(2.3)

where the projected linear separation ℓ of the objects in a binary of inclination i at orbitalpphase φ is ℓ = a (sin φ cos i)2 + (cos φ)2 . Equations 2.2 and 2.3 provide us with theobserved angular separation of two such objects:ℓ H0 (1 + z)
θobs = 2.062648 × 10 ·
c
8

Z z
0


dz ′ −1mas ,E(z ′ )

(2.4)

where c = 2.99792 × 108 m s−1 is the speed of light and ℓ is in meters. If the system is in

32

Chapter 2. Co-detectability of Gravitational and Electromagnetic Waves

a stable orbit, the observed line-of-sight relative velocity of the two objects in a binary isgiven by∆vo = ∆vs sin i cos φ =rGM
sin i cos φ .
a

(2.5)

Binary systems will emit gravitational waves at a frequency fs gw = nfs bin , where n represents the harmonic at which the emission occurs. For the circular orbits to which we limit
our analysis, n = 2; however, n increases for eccentric orbits (Enoki et al. 2004).

2.4 Capability of Electromagnetic Identification Techniques
2.4.1

Direct Imaging of Active Galactic Nuclei

The flat-spectrum emission from radio AGN that is associated with the supermassive blackhole is highly compact, and largely unresolved down to scales of tens of µas. While it waspreviously noted that these cores exhibit flat spectra between 1 and 90 GHz, they will peak
(and, as a consequence, be most readily detectable) at frequencies between ∼2 and 20 GHz.

Radio spectral imaging requires observation at two frequencies, where ν1 < ν2 , preferably
observed simultaneously or near-simultaneously to minimise scintillation and/or intrinsic
variability effects (see §3.3). The resolution of VLBI observations is typically diffraction-

limited, giving a highest observing resolution of θr = c/(Bνobs ), where B is the longest
observing element spacing, and νobs is the observing frequency of the experiment. This
technique likewise requires observations with a sufficient coverage in u − v space that themorphology of the resulting image is representative of the source. Spectral index imaging(again, the techniques of which are discussed in detail in the next chapter) requires taperingof u−v data to match θr of the two frequencies and allow a spectral index map to be made.
As such, the resolving power of this technique—and the limiting factor for detecting thesmall-orbit systems of interest—is θr at the lower observing frequency.
For our computations, we focus on the power of Very Long Baseline Interferometry(VLBI) and in particular the Very Long Baseline Array (VLBA) to resolve components of
AGN to high resolutions. For the VLBA, B = 8611 km, however the effective maximum
baseline depends on a target’s sky position, and decreases most significantly for sourcesof declination δ ≪ 0◦ .

2.4.2

Offset Emission Line Identifiers

There are a number of emission-line identifiers that have been put forward as evidence ofa SMBH binary (including broad lines offset from narrow-line emission, a superimposed2.5. Co-observable Binary Systems

33

set of blue- and red-shifted emission lines in the same series, and AGN line emission witha line-of-sight velocity offset from the host galaxy). Given that the relative velocity of
SMBHs in a pair increases as the binary system evolves—rendering multiple line systems
more readily resolved with optical spectrometers—the identification of offset line systems
associated with small-orbit binaries is not limited by observing hardware.
Instead, the limiting factor for identifying small-orbit SMBH black hole binaries throughoffset-line identifiers is a physical one; for binaries within a sufficiently small orbital separation, the broad-line-emitting regions gravitationally bound to a black hole will be tidally
disrupted by the other. As noted by Dotti et al. (2009), this occurs when the radius of
the broad line region (BLR) associated with a black hole exceeds the Roche Lobe of thatblack hole; that is, rbl < R. Following the Roche Lobe radius approximation of Eggleton(1983), we are limited to
rbl < R =a 0.49 q 2/3.
0.6 q 2/3 + ln(1 + q 1/3 )(2.6)

The size of the BLR can be measured by “reverberation mapping,” which maps the timedelay, τ between changes in the continuum flux in an AGN and the changes in the intensityof a broad emission line in the source. This allows the estimation of the BLR radius asrbl ≤ cτ (e. g. Bentz et al. 2009). Bentz et al. note that cτ is typically on the order of one

light day (≃ 0.8 × 10−3 pc), however rbl in galaxies have been found up to several ordersof magnitude larger, correlated with the continuum luminosity in multiple wave-bands(Kaspi et al. 2005). For simplicity, here we simply set as an average hard limit that thecondition R > 0.001 pc must be met to observe a source. This corresponds to a limit ofa>

0.6 q 2/3 + ln(1 + q 1/3 )pc
490 q 2/3

(2.7)

for the minimum orbital semi-major axis detectable through offset line-emission identifiers.

2.5 Systems Co-observable in Electromagnetic and GW Emission
Pulsar timing and electromagnetic searches for binary AGN are related in two ways.
First, as stressed in this chapter, they can provide interesting scientific results in theco-observation of electromagnetic and GW emission from a binary supermassive system.
Second (stressed more highly in the next chapter), the discovery of binary AGN at various stages of post-merger SMBH binary inspiral can help reveal the currently unknown
timescale taken by the binary to evolve from host-merger to the gravitational-wave dom-

34

Chapter 2. Co-detectability of Gravitational and Electromagnetic Waves

inant regime observable by pulsar timing. This latter aspect is exemplified by Figure
2.2, which indicates the cabalilities of VLBI to resolve a binary radio AGN at various
stages of its inspiral. Within a ≃ 1 kpc, these stages of merger are unresolvable by otherobservational techniques (except for offset-line identifiers, however for these the physicalseparation between two holes is more ambiguous).
As reviewed in section 1.3.1, pulsars have a sensitivity in the gravitational wave spectrum to frequencies 1/Cobs < fo gw < 1/T , where 1/Cobs is the observational cadence (howfrequently one’s pulsars are observed), and T is the length of the pulsar timing data set.
As implied by Eq. 2.1 and the direct connection between the observed gravitational wave
frequency and the binary orbital period, the limits of pulsar timing arrays’ GW frequency
window put bounds on the physical systems detectable by pulsar timing arrays. This isreflected in Figure 2.3, which indicates the systems detectable by the three techniques
considered in this manuscript. For the purposes of this plot we have maximised the linear
projected separation of the SMBH binary (i. e. φ = 0, ∴ ℓ = a).
It is apparent in Figure 2.3 that there is indeed some overlap in the systems theoretically detectable by these techniques, with the most significant intersection between thesystems detectable by pulsar timing and offset line techniques. Note that the BLR limit
shown in this figure is mass-independent, however will increase in a as the mass ratio q
decreases. For pulsar timing, the detectable systems at q < 1 will have smaller orbital
radii; the largest overlap area between these two techniques, therefore, lies in equal-masssystems.
For radio imaging the co-detectable systems are fewer, limited to the most massiveend of equal-mass binary systems at low redshifts: restricted to z . 0.01 for groundbased observations at high radio frequency, and extended to sources of redshift z . 0.1
when using observatories capable of exceptionally long baselines, such as the upcoming
space-based VSOP-2 mission2 .

2.6 Practical Considerations
2.6.1

Prospects of System Co-detection

The probability that a binary AGN has orbital parameters within the bounds of, anddetectable by, the radio imaging or offset-line techniques will depend on a number offactors. This includes the number of (line-emitting/radio) AGN in local galaxies, the rate
1
2

VSOP, see http://www.vsop.isas.jaxa.jp/top.html
VSOP-2, due for launch in 2012; see http://www.vsop.isas.jaxa.jp/vsop2

2.6. Practical Considerations

35

10000
Galaxy merger;
dynamical friction
1000
NGC 6240

Linear scale (pc)100
10Stall region (Yu 2002)
θr = 1 mas
VLBA θr at 22 GHz

Merging of stellar cores"Hard Binary" stage;
3-body interactions
Loss-cone depletion

0402+379

1
D09
B+L09

0.1

4C+22.25

0.01
0.001
0.0001

0.001

0.01
0.1
Redshift

1

10Figure 2.2 The stages of supermassive black hole inspiral as sketched in Fig. 2.1, and an
indication of the parameter space explored by VLBI. The inspiral-stage markings on theordinate indicate a representative semi-major axis for a m1 = m2 ≃ 108 M⊙ paired AGN.
The “stall region” given above represents stalling radii for a range of SMBH masses andgalaxy types, as given by Yu (2002) (see Chap.3). The black encircled points mark the
projected linear separations of proposed SMBH pairs (B+L09 is the system put forward
in Boroson & Lauer 2009 and D09 indicates the system proposed by Dotti et al. (2009)).
Note, however, that the binary SMBH nature of all but NGC 6240 and 0402+379 have
yet to be confirmed.

36

Chapter 2. Co-detectability of Gravitational and Electromagnetic Waves

Maximum projection of semi-major axis (pc)

1000
100
10

θr, VLBA at 8 GHz
θr, VSOP at 1.7 GHz
θr, VSOP-2 at 8 GHz
θr, VSOP-2 at 22 GHz
BLR limit; m1 = m2

1
4C+22.25

0.1
3C66B(proposed but
debunked)

0.01
0.001

m ,m
1
2 =10 10M
m ,ms
1
2 =10 9M un
m ,m
su
1
2 =10 8M n
m ,m
su
1
2 =10 7M n
m ,m
su
1
2 =10 6M n
sun

0.00011e-05
1e-05

0.0001

0.001

0.01

0.1

1

10

Redshift
Figure 2.3 A view of binary AGN that are possible to co-detect through their gravitationaland electromagnetic emission with the Parkes Pulsar Timing Array, offset emission-line
identifiers, and radio imaging (using the ground-based VLBA at its longest baseline at ν1 =8 GHz, and past and future radio interferometers with space-based detecting elements:
VSOP1 operated primarily at 1.7 and 5 GHz, while VSOP-22 is planned to operate at
8, 22, and 43 GHz; baseline length estimated from the satellites’ apogee altitude). The
coloured contours show systems emitting GWs in the pulsar timing band, with a lower
bound at Cobs = 1 wk and upper bound set at T = 20 years. The black points on thisplot give an indication of θr for actual VLBA data (from the data set used in Chapter 3).
The scatter is caused by the range of observational frequencies (v1 = 2 to 24 GHz) andsky positions of the target sources in the data set. The cross labelled “3C66B” SMBH
binary candidate put forward by Sudou et al. (2003) based on abnormal jet motion, andwas subsequently disproven by Jenet et al. (2004) who indicated that the GW emission
expected from this system was not seen in pulsar timing data. 4C+22.25 is an offsetline system which, depending on the uncertainties in SMBH masses and orbital period(reflected by the vertical bar length), may be detectable in pulsar data sets.

2.6. Practical Considerations

Merging pair fraction

0.01

1200

m1,m2 > 1076 M⊙
m1,m2 > 10 M⊙
m1,m2 > 108 M⊙
m1,m2 > 109 M⊙

1000
Number of objects

0.012

37

0.008
0.006
0.004

800
600
400
200

0.002

0.01

All objects
Galaxies
Quasars

0.1

1
Redshift

(a)

10

0
0.0001

0.001

0.01
0.1
Redshift

1

10

(b)

Figure 2.4 (a) The redshift-dependent fraction of merging galaxies (fmrg = Nmrg /Ngal ,representing the number of merging galaxies where m1 , m2 > mlim over the number ofgalaxies at that redshift which host a central black hole of mass > mlim ). (b) The redshift
distribution of radio-emitting galaxies and quasars. This figure reflects all extragalactic
objects of radio flux S > 1 mJy with a measurement of redshift recorded in NED.1
of galaxy mergers in which both galaxies host black holes of m• ≥ 106 M⊙ , and the delay

between host galaxy merger and binary formation (as explored in the next chapter, alongwith the possibility that these systems stall before the stage detectable by pulsar timing).
In Figure 2.4(a), we show the redshift-dependent fraction of virially bound galaxy
pairs, as predicted by the online Millennium Simulation database (see §3.5.4). We use
this here as a representative reference for the SMBH mass-dependent fraction of merging
galaxies in a hierarchical Universe—and thus the fraction of galaxies containing SMBH
binaries, given an efficient post-merger centralisation of the SMBHs.
It is clear that the radio imaging technique is limited to co-detecting systems in only
the most nearby galaxies. This is complicated by the fact that the number of radio-active(esp. flat-spectrum) AGN peaks well-beyond the redshift of co-detectable binaries. To
accent this point, the redshift distribution of radio-emitting AGN down to a flux limitof 1 mJy are shown in Figure 2.4(b).1 Taking these numbers at face value, a flux-limited
search for binary SMBH systems emitting gravitational waves in the pulsar timing band
would be limited to ∼415 targets at z < 0.01, and ∼1950 targets at z < 0.1. This leaves

less than two and eight binary black holes likely to exist in these samples (at z < 0.01 and
0.1, respectively) if the mean merger fraction for radio-active black holes is fmrg = 0.4%.
Given that the flux for many of these sources will be resolved or otherwise very weakat VLBI baselines and the binary systems will have a statistically even orbital phase1

Note: this plot was constructed from data in NED and report only the AGN with a radio flux between
700 MHz and 22 GHz of S > 1 mJy and a recorded redshift; it may suffer minor contamination fromstar-forming galaxies.

38

Chapter 2. Co-detectability of Gravitational and Electromagnetic Waves

distribution, the actual number of detectable targets will in practice be less, leaving little
prospect for success in finding a pulsar-detectable, GW-emitting system in a radio mapping
survey. However, as previously stated, and demonstrated in Chapter 3, such searches cannevertheless contribute to measurements of the SMBH inspiral timescale.
For the offset-line technique, the lack of redshift limit to which the systems are detectable and the non-constraint of radio activity allows the possibility to search of a far
greater number of objects for GW-emitting binaries; the NASA Extragalactic Database(NED) contains over 90,000 catalogued type 1 AGN (that is, Seyfert-1 or Quasars with
detected broad-line emission). Based on the merger rates from the Millennium Database(Springel et al. 2005; see §3.5.4) in Fig. 2.4(a), 300 to 900 of these AGN should be paired
systems. If data were acquired with enough sensitivity to search for, detect and resolve
offset lines in these sources, one would have a strong chance of multiple object detection,even when accounting for the low line-of-sight velocity offsets that some systems will have
due to their phase and inclination orientation at the time of observation.

2.6.2

Sensitivity Considerations for Pulsar Timing

In the sections above we have considered only the frequency range of gravitational waves
explored by pulsar timing. We point out now that in fact some sources emitting in thepulsar timing band may not be detectable because they have too small a GW strain to
induce sufficient power in pulsar timing residuals, such that it is above the noise in thetiming data set. This will have a complex dependence on the GW frequency of the binary,
the constituent masses, the binary’s distance, and the detection method of the pulsar
timing array (take, e. g., the curve of Fig. 1.5 as a reference for one pulsar).
We additionally note that although the overlap between pulsar timing and electromagnetic identifiers increases with longer data sets, the lower-frequency binaries will likewise be
emitting fainter GW signals. Nevertheless, as timing data sets improve their RMS residuallevels and as techniques are developed for using the full power of a timing array—rather
than individual pulsars—to search for and detect GW emission, it is not inconceivablethat one or more co-detectable systems may arise in the coming decades.

2.7 Conclusions
In this chapter we have shown that three direct tracers of supermassive black hole binaries—
VLBI radio spectral imaging, multiple-peaked AGN spectral lines, and gravitational waves—
have overlap in the supermassive binary systems that they are able to detect. Radio

2.7. Conclusions

39

spectral imaging techniques are limited on small scales by the instrumental resolutionof current radio telescopes, giving this method overlap with pulsar timing’s sensitivity
to gravitational waves only at low (z . 0.1) redshift (even with the satellite-based long
baseline interferometry programs past and planned). Offset-line identification techniques,given sufficient integration times, have no redshift-dependent detectability constraints, so
the lower limit on binary separations identifiable using this technique is placed by theradius at which the line-emitting material is tidally stripped by the companion black hole(this should begin to occur at orbital separations of a few milliparsec).
The chance of finding a system detectable through both its electromagnetic and gravitational wave emission will depend highly on the merger rate and the distribution of AGN
in the local Universe, and the amount of time spent in a gravitational-wave-emitting state.
Using simple estimates for the merger rate and distribution of radio- and line-emitting
AGN in the Universe, we find that line-emitting systems should be more successful in
identifying these systems. The identification of co-emitting systems by resolving a double
radio core it is highly improbable, however this technique should still contribute successfully to identifying larger-separation binaries at an earlier stage of inspiral, contributing tostudies of galactic dynamics, merger rates, and other observable signatures of SMBH binaries. The improvements in pulsar timing’s sensitivity to individual GW-emitting systems
that is expected to occur with a dedicated pulsar timing program on the upcoming Square
Kilometre Array should contribute to making the identification of co-detectable systems amore realistic possibility, particularly if it allows a better understanding of low-frequency
timing noise over data spans of 10-20 years.

3
A Radio Census of Binary Supermassive Black HolesA non-detection is still a detection of nothing!
—G. B. Hobbs
In this chapter we carry out a search for binary supermassive black holes, further
developing the radio spectral index imaging technique noted in Chapter 2. While aiming
to detect individual systems co-observable by puslar timing arrays, we also measure thetimescale of post-merger SMBH binary inspiral, a value which was hitherto a significant
unknown, and which contributes directly to the predicted strength of the pulsar-detectablegravitational wave background. We provide the first observational evidence against thetheoretical possibility that SMBH binaries may “stall” at ∼pc orbits—a prospect which,if more than a theoretical fancy, would greatly impact the predictions for PTA-detectablebinary systems and the strength of the gravitational wave background.

3.1 Introduction
Models of the Universe in which hierarchical merging dominates the growth of galaxies havestrong predictions for the presence of binary supermassive black holes at galaxy centres(e.g. Volonteri et al. 2003). As galaxies containing such massive black holes collide, thecentral black holes are expected to inspiral and form a bound interacting system which will
have a significant impact on the central galactic environment (Merritt 2006). Predictionsof the strength of the astrophysical gravitational wave background from SMBH binaries,
and the expected detection rates for LISA and pulsar timing of individual black holes,
are based on parameterisations of merger rates and the SMBH population (Rajagopal& Romani 1995, Jaffe & Backer 2003, Wyithe & Loeb 2003, Enoki et al. 2004, Sesana
et al. 2008, 2009), giving gravitational wave detectors (detailed in §1.3.1) the potential toprovide unique insights into the processes of galaxy formation.
41

42

Chapter 3. A Search for Binary Supermassive Black HolesIn theoretical treatments, however, the evolution of post-merger central supermassive

black holes still holds significant uncertainties. The steps in binary black hole formationand evolution were first laid out by Begelman et al. (1980). After a galaxy pair becomesvirially bound, dynamical friction and violent relaxation drive the stellar cores containing
the massive black holes (of masses m1 and m2 ; m1 > m2 ) to the centre of the merger remnant on roughly a dynamical friction time. When the cores have merged, the black holes
independently experience dynamical friction against the merged core’s stellar environment.
This further centralises each black hole, leading the system to become a bound pair at a
separation where the enclosed stellar mass equals the total binary mass. If the stellar coreis modelled as a single isothermal sphere with stellar density ρ∗ (r) = σv2 /(2πGr 2 ), thisoccurs when the black holes reach a orbital semi-major axis ofabin =

3 G(m1 + m2 )
,
2
σv2

(3.1)

where σv is the velocity dispersion of the merger remnant.1 The length of time spent byblack holes to reach this stage follows the Chandrasekhar timescale for dynamical friction,and is limited by the inspiral timescale for the less massive black hole (cf. Lacey & Cole1993):
tdf = 1.654

r 2 σv,
Gm2 lnΛ(3.2)

where we have assumed circular black hole orbits, and r is the orbital radius of the lessmassive black hole from the centre of the galactic potential. In such a system, we canassume a value for the Coulomb logarithm lnΛ ≃ 5 (a non-circular orbit with ellipticity

ǫ = 0.5 will add a factor of roughly 0.5 to the timescale and the logarithm will be 2 .
lnΛ . 3; e.g. Gualandris & Merritt 2008). The black holes will form a binary whenr = abin .
The orbital evolution of the black holes proceeds further due to dynamical friction,
however as the binary tightens and increases in velocity, dynamical friction becomes anineffective means of energy transfer and the rate of inspiral slows. In this intermediate
inspiral stage, there are significant uncertainties in both inspiral mechanism and timescale.
Relaxing stars in radial orbits entering the “loss cone” of the massive binary (Frank &Rees 1976) will undergo 3-body interactions with the SMBH binary system and be ejected,
carrying away angular momentum and further shrinking the binary orbit. However, without an additional mechanism for energy transfer or a means to efficiently refill the losscone, the binary inspiral halts as the loss cone is emptied. Recognising the long timescale
1

See Sec. 3.5.2 for discussion of typical m1 , m2 , and σv values.

3.1. Introduction

43

that loss cone re-population might take for binary systems, Begelman et al. (1980) suggest
the possibility of gas ejection from the system or gas accretion onto the larger black hole,which will cause shrinkage in the binary orbit to conserve angular momentum (see also,e. g., Merritt & Milosavljević 2005). If some intermediate process is able to sufficiently
shrink the binary orbit, gravitational radiation will cause a binary with ellipticity ǫ = 0
to coalesce in a timescale
tg =

a4
5c5
.
256G3 m1 m2 (m1 + m2 )(3.3)

N-body simulations and semi-analytical models have manifested various intermediate
inspiral processes (see, e.g., the extensive review of Colpi & Dotti 2009). The “last parsec
problem” is the name given to the hurdle encountered by nearly all merger models inwhich the intermediate inspiral mechanisms have timescales sufficiently long that they areunable to bring the binary to a regime in which the emission of gravitational radiation candrive the black holes to coalescence in less than a Hubble time. If this is the case, there
may be many instances of “stalled” binary objects which spend a large portion of their
lifetimes with orbital radii within the range 0.01-10 pc (Yu 2002). A functional form forthe stalling radius is estimated by Merritt (2006):
astallm1 /m2= 0.2.
2 abin(1 + m1 /m2 )2(3.4)

If systems are unable to find the fuel to reach coalescence in less than the age of the
Universe, then they may appear to stall indefinitely at orbits greater (thus GW frequencies
lower, and amplitudes generally smaller) than those in the range detectable by either pulsar
timing arrays or LISA. This would have dramatic consequences for GW detectors and for
hierarchical formation models alike.
Since few small-orbit binary systems are known and electromagnetic tracers of postmerger galaxy cores are hard to identify, it has been difficult to study the post-merger
dynamics of binary black hole systems. To date, there has been no targeted search forsmall-orbit binary black holes. However, as previously noted, several recent searches fordouble AGN have looked for characteristic optical/IR AGN emission lines with doublepeaks or line-of-sight velocity measurements which are offset from the host galaxy, implying
an inspiralling binary at unknown separation (Comerford et al. 2009, Boroson & Lauer
2009; e.g.). The full VLBI Imaging and Polarimetry survey (VIPS, Helmboldt et al. 2007),
has the potential for a large volume of spectral index data, and there are plans to perform
a search of similar method to the one presented in this chapter. Nevertheless, with only

44

Chapter 3. A Search for Binary Supermassive Black Holes

two confirmed sub-kpc double black holes serendipitously identified, (NGC 6240 at ∼1 kpc,

Komossa et al. 2003; 0402+379 at 7 pc, Rodriguez et al. 2006) there is little possibility toassess the physical mechanism responsible for driving binary systems into the phase whengravitational radiation dominates the binary inspiral.

In §2.2, we reviewed several potential electromagnetic identifiers for SMBH binary sys-tems, revealing that taking advantage of these unique spectral signatures of the core regionsof radio-emitting AGN at high radio frequency (1 to 20 GHz) and the sub-milliarcsecond

precision of observations using radio Very Long Baseline Interferometry (VLBI), it is possible to spatially resolve and identify AGN containing multiple cores using multifrequency
gigahertz spectral imaging. This property has also been utilized to locate gravitationallylensed compact radio sources; if an AGN is lensed, it will also appear as a spatially resolved pair with equivalent radio continuum spectra (note that there is direct overlap in
the target of such such searches; because e. g. Wilkinson et al. 2001 found no lenses in 300
VLBI images, this implies no SMBH binaries were found, either).

In the case of a genuine physical pair, resolutions of 1 milliarcsecond are able to discern
components with a projected spacing of above ∼8.5 pc at all redshifts, and down to sub-pc
resolutions for the nearest galaxies (2.2). This makes VLBI observations well-suited to
explore double supermassive black hole systems even at very small projected separations,and within post-merger galaxies that are otherwise unidentifiable as double or disturbed
systems. Identification of double black holes in post-merger systems and binary black
holes in the intermediate stage of inspiral will aid in showing evidence for stalling andmay provide statistical estimates of the rate of SMBH inspiral at various phases.

This paper reports on the first systematic search of a large number of radio sources
for spatially resolved binary radio AGN using archival VLBI data. We present a brief
description of the archival data sample in §3.2, and summarise our search technique in §3.3.

The candidates resulting from this search are assessed in §3.4. We outline an interpretive
framework for making statistical estimates of inspiral timescales based on our detectionsin §3.5, and give the results of this analysis in §3.6. In §3.7 we discuss the analysis in termsof implications for black hole inspiral rates, stalling, radio core emission in the merging
process, and implications for the pulsar-timing-detectable gravitational wave signal from
SMBH binaries.

3.2. The Archival Data Sample

45

Figure 3.1 The redshift distribution of 1575 objects in our sample. The closest source is
at z = 0.000113, while the highest redshift source is a quasar at z = 4.715.

3.2 The Archival Data Sample
The data used in this search were extracted from the VLBI archives maintained online
as the Goddard Space Flight Center astrometric and geodetic catalogues.2 The catalogue
version used was “VLBI global solution 2008a astro,” which contains a large volume ofcalibrated VLBI data from 1980 to early 2008, including data from the VLBA calibrator
surveys (Beasley et al. 2002, Fomalont et al. 2003, Petrov et al. 2005, 2006, Kovalev
et al. 2007, Petrov et al. 2008). The catalogue contained observations unevenly sampledacross 2.2 GHz to 43 GHz for 4169 radio sources. For our search technique, we are limited
to sources observed at two or more frequencies, and to avoid potentially problematic
intrinsic source variability (see §3.3), we rejected frequency pairs which were observed attimes separated by more than 35 days. Our sample was further culled because of software

incompatibility for some data, and for the remaining dataset, observations were rejected if
the images revealed data which contained no clear source above the noise. Three sourceswere removed because a literature search revealed them to be Galactic HII regions or stars.
In total, we were able to search data for 3114 sources.
Approximately half of the sources in our sample had redshift information available.
The distribution for the 1575 sources of known redshift is shown in Fig. 3.1; the source2

http://lacerta.gsfc.nasa.gov/vlbi/solutions/ — Please see website and references therein for a complete
description of original data sets

46

Chapter 3. A Search for Binary Supermassive Black Holes

5
8
15-24
43

2
96
3101
190
34

5
—
94
59
0

8

15-24

43

—
239
37

—
132

—

Table 3.1 Frequency band pairs usable in our search; all frequencies are in units of gigahertz. The frequencies above refer to the rough observing band only. In total there were3982 frequency pairs for 3114 sources.
types and redshifts were gathered from the NASA Extragalactic Database.3 The mean
redshifts of our sample are zg = 0.402, zq = 1.401, and ztot = 1.226 for galaxies, quasars,
and all the sources with known redshift, respectively.

3.2.1

Notes on potential selection effects

The data sets drawn on to create the VLBI global solution 2008a astro catalogue werevaried, however the majority were drawn from geodesy/astrometry or calibrator survey
programs. While the criteria that went into the selection of sources for each constituent
set depends highly on the programs on which the data were drawn (e. g. some were basedon spectral index, some on structural indexes, some on flux density), the consequence
will generally be to bias the sample towards sources that are compact at ≪ 1 arcsecond.

While this should not cause significant bias on the scale of the small-orbit systems ofgreatest interest, it may have a negative effect for the discovery of systems at large orbital
separations (i. e. ongoing-merger or recent post-merger systems). In the analysis of §3.6,
this will bias the measured limits downward, most significantly for “group 1”.

3.3 VLBI Search for Double AGN
Our technique for identifying binary active galactic nuclei using VLBI is essentially a searchfor objects which show more than one flat-spectrum component. It requires morphologicalanalysis and spectral index imaging across two or more frequencies to distinguish thenucleus of an object from its other components, such as complex extended structure or
bright, unresolved hotspots along a jet. These other source components, as was previously
noted, show systematically declining synchrotron spectra at α . −0.7 (note that while we

use the terminology “flat” throughout this chapter, the relevent spectra would be moreaccurately broken down as “flat” and “inverted”, as differentiated from “steep-spectrum
emission” from non-core regions). The technique and the use of archival VLBI data
3

NED, http://nedwww.ipac.caltech.edu/

3.3. VLBI Search for Double AGN

47

require several precautions, primarily because both the accuracy of frequency-dependent
flux density measurements and morphological fidelity of the radio image are important tothis method.
We therefore take special precaution in considering AGN variability and coverage ofthe spatial-frequency (u − v) plane for the data sets. The variability exhibited by someradio AGN typically comes in two different flavours. Both particularly affect compact

regions of emission with high brightness temperature, such as radio cores. The first is
intra-day variability (IDV) due to propagation effects in the interstellar medium. Thissignal flickering can occur on sub-day timescales and exhibits a significant modulationindex at frequencies below 5 GHz, causing fluctuations on average at the 1-10% level (e.g.
Walker 1998, Lovell et al. 2008). Intrinsic variability in radio AGN cores is observed
most significantly at frequencies above 5 GHz, and occurs on a wide range of timescales.
Some radio AGN exhibit slow variations on timescales of years. The existence of suchvariations place limitations on the allowable temporal spacing between observations at
different frequencies. Instantaneous multifrequency spectra will not be influenced by either
effect, while observations spaced at less than the characteristic timescales of long-term
AGN variability will allay the effects of a wandering high-frequency signal. For this reason
we used only frequency pairs with a time separation of less than 35 days. Out of the 3982
usable frequency pairs in our data, 3447 were taken simultaneously, leaving 535 with anaverage difference in observation time of about 11 days.
All data processing was done using the Miriad software package (Sault et al. 1995).
For each frequency pair, we inverted the u−v data, tapering the maxima of u and v to givea circular synthesised beam of the same size at the two frequencies. A manual inspectionof u − v coverage for the pairs ensured that the sources were sampled enough to provide an
accurate representation of the source morphology at the tapered resolution. Images werecleaned, restored with the synthesised Gaussian beam of the tapered resolution to makeimages of 512x512 pixels, allowing a typical field of view 1.2 arcseconds to a side (thoughvarying with the resolution of the observation; this equates to a linear scale at z = 1 of
∼9.5 kpc). An estimate of the image noise level at each frequency was determined using
an off-source region of each image. The number of frequency pairs we were able to use isdetailed in Table 3.1.
The data were imaged in three ways for manual candidate searching. First, for eachresolution-matched frequency pair, the full-field-of-view images at low and high frequency
were plotted in greyscale with logarithmic contours beginning at three times the noise levelof each image. Enhanced plots of the central image regions were made in a similar fashion,

48

Chapter 3. A Search for Binary Supermassive Black Holes(a) J0356+6043(b) J1829+4844(c) J1823+7938

Figure 3.2 Two-frequency spectral index maps for sources tagged as (a) pointlike, (b)resolved, and (c) multiple flat-spectrum component sources. All maps show matchedresolution data. The color scale represents the spectral index calculated from two input
maps of differing frequency. The color scale is linear in greyscale from −1 < α < −0.5,
and linear in color from −0.5 < α < 1. The gray contours show the shape of the source atthe lower frequency used to calculate the spectral index map, while blue contours trace thesource morphology at the higher observing frequency used to calculate the spectral index
map. These sources were randomly chosen from the source list for each category; all show
matched-resolution data between 2.3 GHz (gray contours) and 8.4 GHz (blue contours).
Contours in these plots are set at 2, 25, and 50 % of the peak flux at each frequency.

3.4. Candidates and Search Results

49

and a spectral index map of the inner regions was computed, calculating the spectral index
pixel-by-pixel at any point in the image where the flux at both frequency bands was greaterthan three times the image noise. Spectral index images were superimposed with contours
for each respective frequency, and the spectral index value was plotted as a colour image
with a sharp desaturation break at α = −0.5 to enhance the appearance of flat-spectrum
source components.

We then manually assessed each source, dividing them into pointlike, resolved/extended, and multiple flat-spectrum component categories. Representive sources of eachcategory are shown in Figure 3.2. Sources with multiple flat-spectrum components wereconsidered preliminary candidates. In cases where the sources in multi-frequency images
appeared displaced, a manual best-fit alignment was done and the source was reassessed.
We considered a component to be “flat spectrum” if its two-point spectral index was
α & −0.6 at any frequency pair, or if structure was visible at high frequency which was
not detected at low frequency. As previously noted, sources which contained no detection
above a 4σ noise level were removed from the sample.
The nature of preliminary candidates was then assessed by imaging all available observations of each candidate. Sources were then rejected from the candidate list if theysatisfied any of the following:- In the case of discrete compact flat spectrum components, if suspected components
showed spectral steepening at later observation epochs (implying a false detection
due to a young, evolving jet component).
- If an analysis of the time-dependent movement of suspected flat-spectrum sourcecomponents revealed the component(s) as members of a steady outward (in somecases, superluminal) flow along a jet axis.
- If the full resolution image revealed beam sidelobes as the source of a spurious,
flat-spectrum component.
Our search returned 12 candidates containing genuine multiple flat-spectrum components as discussed in more detail below.

3.4 Candidates and Search Results
Of our 3114 objects, 68% were found to be pointlike at our frequency-matched resolution,
while the remaining sources were identified as resolved or otherwise complex systems
with one flat spectrum component. The number of sources which exhibited multiple

50

Chapter 3. A Search for Binary Supermassive Black Holes(a) Candidate J0541-0541(b) Candidate J1658+0741(c) Candidate J2246-1206

Figure 3.3 Example two-frequency spectral index and contour maps for three of the initial
candidates described in §3.4.1. Coloring of the mapped data and contours are as in Fig.
3.2. Contours are set at 4, 16, 64, and 256 times the RMS image noise drawn from therespective frequency’s input image.

3.4. Candidates and Search Results

51

flat-spectrum components comprised less than 4 percent of the sample, and after theassessment of preliminary candidates, there remained 12 candidate binary sources. As
an example of our candidate data quality, spectral and two-frequency contour images forthree of the candidates below are shown in Figure 3.3. One candidate was the binary
black hole candidate published by Rodriguez et al. (2006), 0402+379; an investigation ofthis source is available in their publication. The remainder of our candidates appear tocontain regions with active jet injection, which effectively gives rise to a population ofcontaminants for the radio spectral index mapping technique, requiring further data andanalysis to distinguish such a source from a binary black hole. Below we use our data andthat available from the literature to further investigate each source and gather evidence
for the presence of such processes, and not a binary black hole, in the sources.
Ultimately, we report the detection of no new binary systems. We have searcheda sample of considerable size, finding only one spatially resolved binary black hole outof 3114 radio-bright AGN. Therefore, within the limitations of our search and data, wefind 0.032% of sources in our sample in which a binary radio AGN was detected. The
implications of this are discussed in later sections.

3.4.1

Candidates with Active Relativistic Injection

Eleven of twelve candidates were rejected after the examination of all available data (inour data, the literature, or available online4 ) revealed that the multiple flat-spectrum
components are in a state of active injection of relativistic electrons in a collimated jet.
These sources can be discriminated from a binary SMBH by several indicators, including
proper (superluminal) motion, or by the resolving of the emission in higher resolution maps
and alignment of the components with a more extensive outflow. Jet components may
exhibit superluminal motion depending on the jet angle, and exhibit spectral steepening as
the electrons age and lose energy; it is furthermore possible that a steadily fuelled outflow
impacting on a dense plasma can maintain a flat spectral index as new relativistic electrons
are constantly introduced. In some cases, we could not dismiss candidates because we
did not have data spanning sufficient epochs to measure spectral steepening or assesscomponent mobility. However, there are two further indicators that distinguish thesesources from double AGN. The first is a morphological trait; by doing component modellingof the source in the highest resolution observation, one can infer the existence of unresolved
components in the source. For flat spectrum components best fit by a Gaussian larger
4

We made use of the MOJAVE database at http://www.physics.purdue.edu/MOJAVE, which is maintained by the MOJAVE team Lister et al. (2009).

52

Chapter 3. A Search for Binary Supermassive Black Holes

than the synthesised beam, it is likely that the flat spectrum emission is due to an active
jet or recently impacted screen rather than direct nuclear emission, which will be compact
and pointlike in our beam even at our best resolution. Secondly, non-AGN core regions ofsynchrotron radio emission in the absence of strong Faraday depolarisation may exhibit
up to ∼70 percent polarisation, typically having between 1 and 50 percent polarisation in

actual observations. Radio cores, however, are often completely depolarised or exhibit lessthan one percent polarisation (e.g. Saikia & Salter 1988, Laing 1988; these effects are also
visible in the flat-spectrum components of the Helmboldt et al. (2007) survey). Thus, even
if a flat-spectrum component is compact, significant levels of polarisation are indicativeof a jet component rather than a radio core. While these sources cannot be ruled out
as binary AGN only by exhibiting one of the above features, These indicators along withthe sources’ morphological traits can provide further insight into and constraints on thenature of each source. The following objects could be rejected in this way:J0111+3906: This source was identified by Owsianik et al. (1998) as an “archetypal
CSO” with a dynamical age estimate of 370 years. Our spectral index maps agree withthe higher resolution ones of Taylor et al. (1996); using their component identifications,
we were able to resolve components C5 and C7 of the eastern jet, the extended C1 western
jet component, and the compact component C3 identified by Taylor et al. as the source’s
core. As in the spectral map of Taylor et al. (1996), ours show flat-spectrum emission
from C3 and the nearby C5. The dynamical age of the source and the extendedness andalignment of C5 along the larger source structure indicate that it is a young jet.
J0541-0541: The most recent (June 2003) epoch shows the double flat-spectrum sourcethat made this object a candidate. While the resolution of our data at earlier epochs is
not good enough to assess component mobility, the southwestern flat spectrum component
contains no compact emission at the high-resolution 8.4 GHz observation, leading to theconclusion that the source is a young core-jet system. The spectral and two-frequency
contour plot for this source is shown in Figure 3.3(a).
J0741+3112: Stanghellini et al. (2005) note that this source is a Faranoff-Riley II quasar
with hotspots on the arcsecond scale and a core-jet morphology on the milliarcsecond
scale, indicating that the source has long been an active core-jet system. There are twoflat spectrum components in our spectral maps, which remain flat over the 10 years thatour data spans. The weaker flat spectrum component lies at the position of a nearly90◦ turn in the jet which Stanghellini et al. interpret as helical jet structure. While ourdata were again not sensitive enough to resolve component mobility, the persistent total
intensity and polarisation observations by Lister et al. (2009) clearly demonstrate that this3.4. Candidates and Search Results

53

source has a continuous outflow which is fuelling the flat spectrum hotspot apparent in
our maps. The northern component in our maps has undetected polarisation at all epochsof the Lister et al. (2009) measurements, further indicating that it is the nucleus.
J1147+3501: This was the weakest candidate, showing a spectral index of α = −0.6

for the southwestern, slightly extended, flat spectrum component. Confirming that thisis a core-jet source are the VLBA observations of Giovannini et al. (1999), which showsuperluminal movement and high levels of polarisation for the western components.
J1223+8040: Morphologically, this source shows a smoothly curved jet over an extent ofapproximately 10 mas. There are several components along the curved jet with α > −0.7
with gradually flatter spectral index and that terminate in the southernmost compact,

highly inverted spectrum component which we tentatively identify as the radio nucleus.
Pollack et al. (2003) indicate that this is a BL Lacertae object, and their measurements of
5-16 percent polarisation tracing a jet along our flat-spectrum components indicate that
this is a core-jet system.
J1347+1217: This radio galaxy has the most extensive (in size) and complex VLBI structure out of the candidates reviewed here, and has 4-5 discrete α > −0.6 components

positioned at various hotspots and bends along its helical jets. While it is possible that anunresolved binary black hole is causing the precessing jet modelled by Lister et al. (2003),
the multiple flat-spectrum components seen in our images all appear to be members of anactive superluminal outflow, as shown by the five year modelling of Lister et al. (2009).
J1459+7140: This source shows a sharp bend in the jet about 50 mas south of thenorthernmost component, turning northeast by approximately 90 degrees. The two flatspectrum components of this source lie in the northernmost hotspot and in a bright knot
midway between the north component and the jet bend. The southern component of
this source, though showing an extremely inverted (α > 1 spectral index, appears at ourhighest resolutions to be fully resolved, while the northern component, at α ≃ 0.2, shows

a compact component with a small-scale jet pointing toward the resolved flat-spectrum
knot. This and the larger-scale 2 GHz emission also lend themselves well to interpretation
as a northern core emitting a possibly precessing southern jet.
J1658+0741: This source contains two flat spectrum objects which, as for J0741+3112,

show little or no spectral change over a span of about 8 years. The weaker flat-spectrum
component of this source is resolved in our non-tapered images and coincides with thelocation of a deviation in the jet alignment of about 30 degrees, as seen in the spectral andcontour plot shown in Figure 3.3(b). If we take the northernmost emission to be the fiducial
point (i.e. the core), we see a new jet emission component which is projected in a direction

54

Chapter 3. A Search for Binary Supermassive Black Holes

directly incident on the component at the jet bend. The trail of emission between thisand the southeastern component, the non-compactness of the northern component suggestthat the hotspot is at the location of a dense screen which is deflecting the relativistic jet,causing its reorientation. These morphological clues suggest that the source is not a binary
AGN.
J1823+7938: There are two key indicators that imply the southwest component is a
nucleus while the northeastern component is not. First is the resolved structure of theeasternmost component in our data in our high-resolution data at 8 GHz and second theproper motion analysis of Britzen et al. (2008) show a hub of jet activity surrounding
the southwest component and find proper motions of the hotspots between our two flatspectrum components receding from the southwest core in the direction of the brightemission to the northeast. It is clear from the separation between the two flat spectrum
components that if the northeast hotspot originated from the southwest core, its emission
was not recent. The morphology of the northeast component, however, is reminiscent of acompressed shock region; polarisation studies of this source could clarify the mechanisms
at work in this source, however the evidence present here is enough to indicate that it is
not a binary radio nucleus. This candidate is shown in Fig. 3.2(c).
J2246-1206: This source has two components of α > 0.3 connected by a faint string ofemission, with a single stream emanating north-east from the northern component. Whilesuggestive of two cores (one associated with a single-sided jet), by modelling the relative
movement of the northern component, the movement is neither consistent with binary
motion relative to the southern component, nor is it aligned to the motion of the jet
components (Lister et al. 2009). We conclude that this is yet another source in which
there is a core (the southern component) actively emitting a jet, which upon impact witha screen is deflected and reoriented. This candidate is displayed in Figure 3.3(c).
J2253+1608: This source has two flat-spectrum hotspots traced by a steep-spectrum jet.
The westernmost hotspot, like in the case of J2246-1206 and J1658+0741, sits in theposition of a jet bend. The careful monitoring of Lister et al. (2009) show the secondary
flat-spectrum component to be a jet member.

3.5 Framework for Interpretation
The use of radio data as a black hole indicator puts immediate constraints on both thephysical properties and number of sources we will be sensitive to. We adopt the following

3.5. Framework for Interpretation

55

expression for the number of sources expected to be found in our search:
Nexp =

Ns
X
i=0

fbbh (zi , mlim ) · Pi

(3.5)

Here, Ns is the number of objects that have been searched. It is assumed that a target’s
redshift z is known (true for 1575 of our objects), and that it contains at least one detectable black hole of mass M• > mlim .5 P represents the probability that we would detect
a second black hole if the system is a binary, being the probability that the secondary blackhole is both radio-emitting, and bright enough for a successful detection in our images.
This term is explored in Sec. 3.5.1 below. The fraction of galaxies, fbbh , containing a
SMBH binary system at a redshift z ′ to which we are sensitive will be the cumulative
number of galaxy mergers over the epoch at z ′ until the epoch at z ′′ [tz − tvis ], where tz isthe age of the universe at redshift z ′ , and tvis is the duration that a signature of a double
black hole in the merging system is detectable in our search. Thus, if a binary stalls at a
separation detectable by our technique (bounded by our observing resolution and the fieldof view of our images; see §3.5.3), galaxies that began to merge when the Universe wasthe age of z ′′ will still be occupied by a resolvable binary at z ′ . The occupation fraction
for binaries at a resolvable separation is then given by:fbbh (z, mlim ) =R z ′′z′

Nmrg (z, mlim ) dz,
Ngal (z ′ , mlim )(3.6)

where Nmrg is the number of virialized galaxy pairs with SMBHs of masses above mlim
at the given redshift, while Ngal is the total number of galaxies at z ′ with SMBH masses
greater than mlim. The timescale of inspiral over the scales to which our data are senstive(tvis ) can thus be estimated with a prediction or measurement of the redshift-dependent
merger rate of galaxies containing supermassive black holes.
Whether we can identify a binary depends on factors explored in the sections below.
The timescale over which we can resolve a binary is determined by our spatial sensitivity,
the black hole masses and binary mass ratio; the relative time spent at various separations
in a binary’s inspiral depend on black hole mass. If the radio luminosity of the black
holes somehow depends on the stage of the merger (for instance if the radio mechanism
5

Throughout our analysis we use a value mlim = 108 M⊙ ; here we make use of the observed property
that radio-bright quasars have a distinct mass distribution: hlog(M• /M⊙ )i = 8.89 ± 0.02 with a lower
cutoff around 108 M⊙ (e. g. McLure & Jarvis 2004). We probe only this most massive end of the blackhole population, and for a double detection to be made, the second black hole must exceed a limiting mass
mlim ≥ 108 M⊙ .

56

Chapter 3. A Search for Binary Supermassive Black Holes

shuts down as the galaxies begin interacting, or ignites only after a binary coalescence),an additional factor needs to be considered in Eq. 3.5. However, because the evidence
for a relationship between radio fuelling and merger events has until now been tenuousand estimates for an AGN timescale are so far inconclusive, we limit this uncertainty byemploying two scenarios below that represent the most optimistic and the most pessimistic
expectations for the state of radio luminosity in the two black holes.

3.5.1

Supermassive Black Holes and Radio Emission

All our targets are radio sources detectable on parsec scales, and it follows that all containat least one radio-emitting supermassive black hole. If a target is a binary system witha resolvable separation between the two black holes, due to our inspection techniques asecond radio-emitting SMBH will have only been flagged as a candidate if its flux density
exceeds four times the root-mean-squared noise (σrms ) in its image. This 4 σrms flux limit
corresponds to a luminosity sensitivity limit of Llim (σrms , z). To predict the expected
number of binary AGN to be found in our search and estimate a timescale for binary
SMBH inspiral, we must first quantify the likelihood that the radio emission of a second
black hole will exceed this luminosity limit.
The main unknown contributing to the discussion is how strongly the galactic andintergalactic environment relate to the production of radio AGN activity around the central
black hole: that is, do the merger and mixing of gaseous, dusty, and stellar environments
contribute strongly the production of radio activity? Below we assign two scenarios tobound assumptions at either end of extremes: 1) Pessimistic; the probability that a SMBH
paired with a radio-emitting AGN is itself radio-emitting is no greater than the probability
that a non-binary SMBH at the center of a massive galaxy would be radio-emitting, and
2) Optimistic; given a black hole that exists in a shared environment with a radio-emitting
AGN, the probability of radio emission in the second black hole is unity.
Given that the black holes in the progenitors may not encounter significantly mixed
environments until the binary has a relatively small separation, it is also possible that radio
ignition is radially dependent. Regardless, we stress that for the third stage of inspiralin which theoretically the binary may stall, the stellar cores around the black holes havemerged and the black holes will have shared many orbits within one another’s sphere of
influence and in a common environment.

3.5. Framework for Interpretation

57

Pessimistic scenario
We can make an estimate of the fewest number of binary sources we would expect to see by
setting P equal to the integrated bivariate luminosity function of galaxies, ψ(M, >Llim ),where M represents the absolute optical/infrared magnitude of a galaxy. In this case
we assume that mergers have no influence on the radio-active state of a black hole; thatis, that the probability that the second black hole in a binary would have a detectable
luminosity L > Llim is equivalent to the radio active fraction for non-merging galaxies
selected from a random pool with M• ≥ mlim . We calculate P for each source using a

log-linear interpolation of the integrated bivariate luminosity function of Mauch & Sadler(2007) for radio AGNs in galaxies with −24 > MK ≥ −25, taking such galaxies to be
representative of our targets’ host galaxies. As roughly 10% of the Mauch & Sadler (2007)

sample have flat spectra (based on the measurements of Mason et al. (2009)), we expectthat approximately 10% of their sources will be compact down to VLBI scales. To accountfor this we divide the space density given in their radio luminosity function by a factorof 10. For our 1575 sources with a known redshift, we find that in this scenario P has amean and standard deviation of 0.4% and 1%, respectively.
Optimistic scenario
In the optimistic scenario, we consider that a SMBH in the presence of a radio AGN willitself be a radio-loud AGN. This scenario implies that when the two black holes share acommon galactic environment, both black holes will be radio luminous if the conditions
are fit for radio ignition; that is, both objects will be either radio-loud or not. We takeas a given that one black hole is a detectable radio AGN in each of our images. To
determine the detectability of a second SMBH in such a system (the probability that its
observed luminosity is greater than Llim ), we would like to avoid the explicit use of e. g.
relativistic beaming models, correlations between black hole mass and radio luminosity,and the levels of flux resolved by our baselines—the parameters of which are uncertain—by
using an estimate derived from observations in which such effects are inherent.
We therefore take the secondary SMBHs in such systems to have a radio luminosity distribution equal to the luminosity distribution of solitary, flat-spectrum radio black holes,
which will implicitly include beaming and mass-correlation effects. The use of the flatspectrum population assumes that the emission observed in flat-spectrum radio sources islargely unresolved, and therefore scalable to the resolutions reached by our VLBI data. We
adopt a probability density function for secondary black hole radio luminosities, ρ(z, L) dL,
based on the redshift-dependent, flat-spectrum “pure luminosity evolution model” of Dun-

58

Chapter 3. A Search for Binary Supermassive Black Holeslop & Peacock (1990). We may then determine the probability that for a given object inthe sample, a second black hole will have a luminosity above our detection threshold, Llim .
The probability that we would have detected the second black hole of a binary AGN is
therefore:
P =The normalisation factor, ρn =

1
ρn

Z ∞

ρ(z, L) dL .

(3.7)

Llim

R∞

LRL ρ(z, L) dL, sets the range of radio luminosities ofradio-loud AGN. We use a lower luminosity bound of LRL = 1023 W Hz−1 sr−1 for radio

AGN, equal to that given by Padovani et al. (2009). Although the exact value of LRL
debatable, the average P value is not highly sensitive to the value of ρn if LRL is decreased.
For images in which the Llim < LRL , the probability of detection is set to one. For the1575 known-redshift sources in this scenario, P has a mean and standard deviation of
57.9% and 23.7%, respectively.

3.5.2

Host Properties and Limiting Radii

The predicted binary formation radii and stalling radii mark key points in possible inspiralrate changes for a post-merger system. In order to determine what stage of inspiral we haveprobed with our spatial sensitivity limits, we require an estimate of the predicted typicalabin and astall for our sample. As mergers between less massive galaxies are expected
to be more common than high mass mergers, for this study we expect the least massive(and most common) merger probed in our sample to involve two black holes of 108 M⊙ .
Relevant progenitor galaxy properties can be calculated to estimate the characteristic
scales (e.g. stalling radius, binary formation radius) that our study probes. Using two
black holes of mass 108 M⊙ , each progenitor’s velocity dispersion, σv ∼ 187 km s−1 , andbulge mass, Mhost ∼ 6.63 × 1010 M⊙ , can be estimated by the M• − σv relation of Ferrarese& Merritt (2000) and the M• − Mbulge relation of Häring & Rix (2004), respectively. If
the black holes do not accrete a significant fraction of their mass during inspiral, the postmerger galaxy will contain a black hole of 2 × 108 M⊙ and an implied velocity dispersion ofσv ∼ 216 km s−1 . The binary formation radius for our typical system is thus abin = 27.7 pc,

while the stalling radius is astall = 2.8 pc (equations 3.1, 3.4). In the earliest stage ofmerger, the virially bound galaxy pair will have a projected separation . 7 kpc. We notethat while these radii are the characteristic values for the sources in the sample, more
massive (and less common) mergers between 109 M⊙ black holes will have larger radii bya factor of ∼10.

3.5. Framework for Interpretation

59

10000

linear scale (pc)

1000

100

10

1
<Dmin><Dmax>

0.1
0.01

0.1

1
redshift

Figure 3.4 The upper and lower bounds of the average linear scales searched as a functionof redshift. The error bars represent the standard deviation of hDmin i and hDmax i.

100

Dmin (pc)

10
1
0.1
0.01
astallabin

0.001
0.0001

0.001

0.01
0.1
redshift

1

10Figure 3.5 The minimum resolvable linear scale for each of the sources of known redshift inour sample. Overplotted are the predicted black hole binary formation radius and stallingradius for two 108 M⊙ black holes. Note that for a binary in a circular orbit, these radii
correspond to a maximum projected separation of twice each value.

60

Chapter 3. A Search for Binary Supermassive Black Holes

3.5.3

Spatial Limits of the Search

Our technique resolves a large range of spatial scales, over which an inspiral rate will
undergo several changes. To estimate post-merger black hole evolutionary timescales wemust determine the projected spatial separation range that our data set is sensitive to. For
the 1575 sources of known redshift, the average maximum and minimum projected linear
spatial sensitivities (hDmax i, hDmin i, both in units of parsec) as a function of redshift areplotted in Figure 3.4. The sources are binned in increments matching the redshift steps of

the Millennium Simulation snapshots (see §3.5.4; each step spans an average of 300 Myr).
The observed scales are derived as follows: the minimum and maximum linear projected separation between two objects that we are sensitive to are set by our angularresolution, θr , and half the field of view of our search maps, given that one AGN will lie
at the pointing centre of the observation. Using the redshift of each source and a flatuniverse cosmology with H0 = 72 km s−1 Mpc−1 , Ωm = 0.27, and ΩΛ = 0.73, we calculate
dz /θz , the distance (in pc) per milliarcsecond at that redshift on the plane of the sky. Our
smallest resolved size for that source is then Dmin = dz θr /θz parsecs (Dmin for individual
sources are plotted in Figure 3.5). Because we searched images of 512 pixels to a side andthe synthesised restoring beam is 3 pixels in breadth, the maximum linear scale we aresensitive to is:
Dmax =

512 θr dz·
parsecs .
6
θz

(3.8)

This value represents the largest scale size to which the search of each source is complete.
For sources with known redshift, the sample averages were hθr i = 6.5 mas, hDmin i =40 ± 25 pc and hDmax i = 3415 ± 2133 pc.

The limited spatial separation window over which we have searched each source for a
double AGN and the potential for drastic changes in inspiral rate at various stages of thebinary evolution compel us to divide our measurement of inspiral rates into three stages.
We do so by segregating the sources by their spatial resolution into groups which represent
sensitivity to three stages of merger evolution: the stage at which the stellar cores have
not yet merged (merger stage 1), the stage leading to binary formation in which thecores merge and the binary forms (stage 2), and the final stage to which we can probespatially, in which the black holes are a binary and evolve to the predicted stalling radius(stage 3). In each of these groups, we consider that a linear resolution of Dmin = alo willallow the detection of sources with an orbital semi-major axis of alo roughly 80% of thetime, assuming systems with a random distribution of inclination angles. By setting theconstraint on a group that Dmin < alo , we ensure that the searched nuclei in that group3.5. Framework for Interpretation
0.1

61

5 Gyr

Binary occupation fraction

0.01

fmrg
fbbh

13.7 Gyr

3 Gyr
1 Gyr
0.5 Gyr
300 Myr

0.001

100 Myr

0.0001
1 Myr

1e-05

1e-06
0.01

0.1

1

10

redshift

Figure 3.6 The redshift-dependent fraction of galaxies containing binary or double SMBH
black hole systems for various inspiral times, based on the merger rate predictions of De
Lucia & Blaizot (2007) for mergers with black holes of masses m1 , m2 ≥ 108 M⊙ .
will be sensitive to a binary with axis a ≥ alo for > 80% of the pair’s orbit. Each groupis statistically sensitive in this way to pairs with an orbital axis of up to hDmax i of thegroup.

The three groups are divided at (1) 27.7 < Dmin < 120 pc, (2) 2.8 < Dmin < 27.7 pc(sensitivity down to our limiting abin ), and (3) Dmin < 2.8 pc (giving sensitivity down to
the minimum astall ). Because sources with very high resolution observations will not havea large value of Dmax and thus not probe large separations, the source group (3) is notincluded in (2), and in turn these two groups are not included in (1). The divisions givea sensitivity to binaries at separations over three ranges of scales:(1) 120 < a < 4757 parsecs(2) 27.7 < a < 2519 parsecs(3) 2.8 < a < 450 parsecs
There were 1035, 497, and 43 sources in groups one, two, and three, respectively.

3.5.4

Galaxy Merger Rates

The merger rate of galaxies containing SMBHs given a ΛCDM universe in which merging
is the primary mechanism for galaxy growth can be predicted using results from the62

Chapter 3. A Search for Binary Supermassive Black Holes

Millennium Simulation (Springel et al. 2005). The Millennium simulation is a large-scale
N-body simulation which tracked the evolution of dark matter halos in a co-moving cubic
volume 500 Mpc h−1 to a side (h = H0 /100 km s−1 Mpc−1 ). De Lucia & Blaizot (2007)
applied a semi-analytical prescription to the Millennium simulation to track the evolution
and merger histories of individual galaxies within the Millennium volume. Their catalogue
can be queried through an online interface.6
Using the Millennium catalogue, we constructed the redshift-dependent distributionof galaxies and mergers in which each galaxy contains a supermassive black hole ofM• > 108 M⊙ . For comparison with the black hole binary occupation fraction, we showthese values as a pair fraction, where fmrg = Nmrg /Ngal ; here, Nmrg and Ngal are thenumber of mergers and the total number of galaxies, respectively, with M• > 108 M⊙ at
the given simulation snapshot. A “merged” pair in the Millennium Simulation and the
De Lucia & Blaizot framework is a pair which has become gravitationally bound in the∼300 Myr between two adjacent snapshots.

Figure 3.6 shows the redshift-dependent pair fraction for mlim > 108 M⊙ and the cor-

responding black hole binary occupation fraction (computed from eq. 3.6) for a range ofinspiral times. The merger distribution peaks at a redshift of about z = 0.7.

3.6 Limits on Inspiral Timescales for SMBH Binaries
Following Eq. 3.5, for the merger stage groups with no SMBH binary detections, we place
an upper limit on tvis at Nexp = 0.5. The one source in which a binary black hole was
detected is part of the second merger stage group; for this group/region only are we able
to make a measurement of (rather than put a limit on) tvis , with errors bounding wheretvis gives a value of 0.5 < Nexp < 1.5.
The timescale measurements for the optimistic scenario are summarised in Figure 3.7,
which illustrates the number of black holes expected to be found in each group as afunction of tvis , and Table 3.2, which gives the numerical values of our limits. Under the
pessimistic set of assumptions, we would not expect to see any sources, given that evenwith tvis = 1/H0 , we find Nexp = 0.025, 0.20, and 0.16 for groups 1, 2, and 3, respectively.
The implications of this will be discussed in the next section.
In Table 3.2 we report the timescales measured from both our 1575 sources of known
redshift, and from the full sample with an assumed redshift distribution. While we do
not use the less reliable “full sample” timescale values in the analysis below, we stress thelimiting power of the sample if we were to have a full sample with measured redshifts, by
6

See http://www.g-vo.org/Millennium

3.7. Discussion

63

12
10

Initial inspiral (1)Merging core region (2)Bound binary + beyond
stalling radius (3)

Nexp

8
1/H0

6
4
2
0
100

101

102
103
tvis (Myr)

104

105

Figure 3.7 The cumulative expected number of sources and limits on the inspiral timescalefor each spatial-sensitivity limited group using the optimistic scenario (§3.5.1) in which
given one radio-active black hole in a binary, the second will also be radio-emitting. For
groups 1 and 2, the upper limit set on tvis is marked with an arrow, while for group two
the measured value of tvis is marked by a filled circle, with open circles indicating the error
range.
assuming the remaining sources have a redshift distribution equivalent to the other 1575.
This has the effect of roughly doubling Nexp for each group, and as such works only to lay
more stringent limits on tvis .
Finally, although the number of sources in group one was the largest, these sources
also had a tendency to be of higher redshift, with a net effect of fewer expected detectablebinary black holes due to the lower merging fraction of SMBH hosts at redshifts z & 1. For
stalling studies, the observation of low-redshift sources is therefore doubly beneficial: notonly does it allow smaller scales to be resolved in the sources due to their proximity, but it
affords a higher predicted success rate due to the increased merger rate of SMBH-hosting
galaxies at low redshift.

3.7 Discussion
The question we aim to answer with this search is whether there is observational evidence
for stalled supermassive binary black hole systems: that is, what is the evolution timescale
down to the stalling radius for a post-merger supermassive black hole pair? For our
methods, this question is intertwined with how closely correlated the radio emission is

64

Chapter 3. A Search for Binary Supermassive Black HolesGroup 1
Group 2
Group 3Net

Known redshift<0.1190.286+148−143<0.846<1.25 Gyr

Full sample<0.0600.143<0.416<619 Myr

Table 3.2 A summary of the tvis measurements (in Gyr) placed by our known redshiftsources (1575) and the full sample with an assumed redshift distribution (3114 total). The
values given are for the set of optimistic radio luminosity assumptions (§3.5.1). Groups
1–3 are as detailed in §3.5.3. Although the groups overlap in their linear separation
sensitivities, we quote a “Net” timescale that gives a limit on the total timescale of inspiralfrom galaxy virialization to SMBH separations beyond the stalling radius.
between two black holes in a pair. Our sample allows us to address this correlation andinspiral timescales over three stages of post-merger SMBH pair evolution. This is discussed
below, followed by a brief consideration of the consequences of our results for the detectionof gravitational waves from SMBH binary systems by pulsar timing arrays.

3.7.1

Post-merger Efficiency, Radio Ignition, and SMBH Binary Stalling

The timescales measured for our three merger stages represent the time a pair’s orbital
separation takes to evolve through the spatial sensitivity window of each stage. For ourmerger stage groups one and two (∼virialisation down to SMBH binary formation), it
is expected that purely dynamical friction will act on the black holes. Formally, thedynamical friction timescale in equation 3.2 over the range probed by our first and second
merger stage groups is ∼3.7 Gyr and 900 Myr, respectively, for an object of mass 108 M⊙ .

For merger stage group one, the optimistic scenario timescale limit is smaller than the

dynamical friction timescale by a factor of ∼30. From this disagreement, it follows that

one of the following must be true: 1) the relative radio state of two SMBHs in merging
galaxies is not strongly correlated before the hosts’ stellar cores have merged, and/or 2)
The centralisation of supermassive objects in a post-merger system is highly efficient (inexcess of dynamical friction).
Considering merger stage group two, in which we have detected one binary SMBH, thetimescale under the optimistic set of assumptions gives a value more consistent (a factorof ∼2 within the error bounds) with dynamical friction. Because in the pessimistic limit

we would not expect to see any sources, it is implicit that the pessimistic assumptions are
not strictly valid for this stage of merger. This is furthermore indicative of some radial
dependence of the relative radio state of two SMBHs in a post-merger environment, suchthat the SMBHs have a correlated radio active state at later stages of SMBH inspiral,

3.7. Discussion

65

after the pair has shared a common galactic environment for many orbits.
We have set the cutoff for the third merger stage group to be sensitive to the theoreticalstalling radius of Merritt (2006). While the limits that we can place on this stage of merger
are the least stringent due to the small source count in the limiting group, they represent
the first observational exploration of stalling. If the evidence exhibited by merger stage
group 2 holds—that the relative radio state between two black holes is more strongly correlated at late stages of binary evolution—our results are in agreement with a progression
through the stalling radius in much less than a Hubble time; that is, we find no evidence
for systematic stalling of supermassive binary black hole systems. These results, however,exist with the significant caveat: if the most pessimistic assumptions are true, we would
not expect to see any paired SMBHs regardless of the length of post-merger inspiral.
As previously noted and reflected in table 2, we have only used the information fromroughly half of our sources to acquire timescale measurements. The lack of detections in
the additional 1539 sources suggests that the measured limits will only be more stringent,
however would be highly dependent on the redshift distribution of the remaining sources.

3.7.2

Consequences for the GW Background and Pulsar Timing Arrays

The measured inspiral timescales and lack of evidence for stalled supermassive systems
are pertinent results for pulsar timing arrays. While we have primarily accentuated thatthe brevity of inspiral timescale implies an efficient passage through the stalling stage,
we note also that the timescales measured for merger group two (in concert with timespent at other separations) are sufficiently large that they may have non-negligible effects
on predictions of the gravitational wave background signature in the local Universe from
SMBH binaries. The GWB prediction models discussed in §1.3.1 typically assume efficient

or instantaneous inspiral into the gravitational wave regime, equating redshift-dependent
galaxy merger rates with binary coalescence rates. The inspiral times measured here
may cause the galaxies merging at a redshift of 0.7, at which the merger distribution
peaks, to emit gravitational radiation at a much later epoch, causing a larger expected
amplitude for the astrophysical gravitational wave background than previously estimated(recall equations 1.11, 1.12). In addition, it is likely that this effect would cause an increasein the predicted number of supermassive binary sources whose gravitational radiation isstrong enough to be resolved from the gravitational wave background by pulsar timing
arrays (Sesana et al. 2009).

66

Chapter 3. A Search for Binary Supermassive Black Holes

3.8 Summary
We have performed a search of 3114 active galactic nuclei for the presence of double SMBH
systems using a multi-frequency radio imaging technique. Of the sources searched, only
0402+379 was apparent with our method as a binary AGN. While this source has alreadybeen put forward as a binary black hole in Rodriguez et al. (2006), this search represents
a significant enough statistical sample with which to interpret the existence of this sourcein the broader cosmological context of binary supermassive black holes.
This search has probed estimates of the inspiral timescale for binary black holes of
m1 , m2 ≥ 108 M⊙ . For the 1575 sources that had redshift information with which todiscern the linear scales probed in the host galaxy, we obtained timescale estimates forthe inspiral of binary systems in three ranges of binary separation. We have demonstrated
observational evidence against stalled binary black hole systems by demonstrating that: 1)
at late stages of SMBH pair inspiral, the two black holes are more likely to be in a similarly
radio-active state, and 2) SMBH binaries proceed from separations of a ≃ 500 parsecs

to within the stalling radius estimates of Merritt (2006) in less than ∼0.5 Gyr. The
implications of this are that supermassive binary systems do not stall indefinitely at suchradii, suggesting there is a yet undetermined mechanism by which the black holes are able
to dispense energy to the surrounding environment and proceed to coalescence.
The results of this study come with the caveat that they stem on very small-number
statistics; that is, we detected only one binary AGN in the analysis, and due to unmeasured
redshifts, we were unable to use roughly half of the sources searched in the statisticalanalysis. Definitive studies of this type for large numbers of AGN would become possible
with the large collecting area and dense instantaneous u−v coverage of the planned Square
Kilometer Array (SKA). However, as implied by Eq. 2.4 and the analysis of Chapter 2, theSKA will only be a productive research instrument for this method if long baselines areincluded in its configuration (e. g. baseline length B & 1200 km at 5 GHz would give thecapability of resolving the stalling regions of the most massive supermassive black holepairs at z < 0.01).

4
An Archival Search for Radio Transients:
Implications for RRATs and Extreme Pulsar
Intermittence

The two prime movers in the Universe are Time and Luck.
—K. Vonnegut Jr.
In this chapter we develop a single pulse search methodology which makes novel additions to the search processes reviewed in §1.4.2. We then explore the nature of RRATs

through fourteen new discoveries of sparsely-emitting neutron stars in a blind search fortransient radio bursts of between 0.125 and 32 millisecond duration. The new detections

provide key insights into RRATs and extreme nulling phenomena in pulsars, and support
the premise that RRATs may simply be nulling pulsars that are only “on” for less than apulse period. The newly discovered object PSR J0941–39 may futhermore represent a direct link between pulsars and RRATs, as it appears to oscillate between a sparsely-emitting“RRAT” state and a ∼10% nulling-fraction “pulsar” state.

4.1 Introduction
As reviewed in §1.4.2, the transient radio sky has revealed many classes of exotic objects
since the discovery of pulsars in late 1967 (Hewish et al. 1968), with the recent discoveryof sparsely bursting rotating radio transients (RRATs, McLaughlin et al. 2006), and aone-off burst tentatively identified as having an extragalactic origin (Lorimer et al. 2007)
spurring renewed interest in the search for single-pulse emission after 30 years of Fouriertransform searches dominating surveys for pulsars.
67

68

4.1.1

Chapter 4. Transients in Archival Pulsar Surveys

A Brief History of Single Pulse Searching

The more recent history of single-burst searches commenced with the search of Amy et al.
(1989) for impulsive signals between 0.001 and 800 ms using a transient event monitoring
system at 843 MHz at the Molonglo cylindrical telescope. While they found only pulsars
and ambiguous events, they were able to put an upper limit on the event rate of 1 − 25 ms
duration events of 0.017 events s−1 sr−1 for pulses with energy density > 10−28 J m−2 Hz−1 .
Nice (1999) also recognised that some pulsars might be undetectable in periodicity searches
and subsequently re-searched data from a 68 deg2 area of sky along the galactic plane forsingle pulses with duration between 0.5 − 8 ms, revealing one new, slow (2.3 s) pulsar.
The survey which discovered RRATs (McLaughlin et al. 2006; hereafter M06) was thefirst large-scale examination of a significant data set for impulsive bursts since the early
pulsar surveys, while the search of the Magellanic Cloud surveys (Lorimer et al. 2007) anda transient search with Arecibo telescope (Deneva et al. 2009; hereafter D09) and a deeper
search of the M06 data (Keane et al. 2010; hereafter K09) followed using similar techniques.
A large volume of archival pulsar surveys remain to be searched, however, which may
contain single events from yet unidentified phenomena, and will certainly contain further
examples of RRATs. Higher sensitivity to rare events in future searches will benefit fromfurther sophistication in radio interference mitigation, which we attempt to address in our
search presented below, and from alternate methods of data inspection, which we also
address with our search below.

4.1.2

The Ambiguous Nature of RRATs

At present it is unclear from the few known RRATs how they relate to the neutron star
population as a whole, or how they may relate to other sub-populations of neutron stars
such as anomalous X-ray pulsars or magnetars (recall Figure 1.2, in which the positionof several RRATs in the P − Ṗ diagram appeared to lie midway between magnetars andpulsars. If RRATs are not a distinct class of life-long sparse emitters, it may be that thesparse emission of RRATs represents a temporary phase through which all pulsars undergo
when they reach a sufficient age (e.g. McLaughlin et al. 2009, Keane & Kramer 2008), orthat are simply distant, low-luminosity pulsars with extreme pulse amplitude modulation(Weltevrede et al. 2006b). Such distinctions are important for reconciling the so-called“birthrate problem”; it has been recently noted that the net neutron star birthrate calculated from apparently disparate neutron star populations (X-ray dim isolated neutronstars (XDINS), magnetars, RRATs, pulsars) exceeds the rate of the core-collapse supernovae that are believed to produce them (Popov et al. 2006, Keane & Kramer 2008).

4.2. Data Sample

69

Recent works are endeavouring to perform analyses that make the distinction clearer frompulse timing (e.g. McLaughlin et al. 2009). Furthermore, continued searches using singlepulse search algorithms promise to reveal further examples of sparsely-emitting neutronstars, providing a larger statistical sample over which to assess the physical nature of thesesources. The discoveries presented in this chapter provide key insights into the relationshipof RRATs and steadily-emitting pulsars.
In this chapter, we present the discovery of fourteen new neutron stars, resulting froma blind search for single pulses in more than 900 hours of archival data from the pulsar
surveys of Edwards et al. (2001) and Jacoby et al. (2009). Our data is described in §4.2,

and the new search methodologies used in this search are detailed in §4.3. The fourteen
new objects and other results uncovered by this search are presented in §4.4. We explore

the nature of RRAT and deep-nulling pulsars, compare the properties of radio neutronstar populations, and discuss the implied consequences for the origin of RRAT emissionin §4.5. Several other important discoveries made in this search are presented in Chapter
6.

4.2 Data Sample
The data used in this search were the southern-sky (−100◦ < l < 50◦ ) intermediate(5◦ < |b| < 15◦ ) and high (15◦ < |b| < 30◦ ) galactic latitude surveys of Edwards et al.

(2001) and Jacoby et al. (2009), respectively; here b and l represent galactic latitude

and longitude, respectively. Both surveys employed the same analogue 1-bit filterbank asa back-end to the 20cm multibeam receiver system on the Parkes Radio Telescope. The
multibeam receiving system allows the simultaneous observation of 13 positions on the sky
with approximately 30 arcminutes between beam positions (Staveley-Smith et al. 1996).
The twelve outer beams are arranged in two concentric sets of six (see the upper left panelof Fig. 4.1), with a full-width half power beamwidth of each beam at 14 arcminutes. Dueto the sharp sensitivity fall-off within 24 arcminutes of each beam’s pointing centre, apointlike astrophysical signal will generally be detected in no more than four beams of thereceiver, and unless particularly bright, usually just one or two.
Each survey sampled 96 channels across a 288 MHz band centred on a frequencyof 1372.5 MHz. The data were recorded after the polarisations were summed and thensampled at 125 µs. Each 13-beam pointing of the telescope lasted 4.4 minutes. The data
total 12,090 beam hours (there are 13 beams), taken over the years 1998-1999 (Edwards)
and 2001-2002 (Jacoby). The total number of pulsars detected in the periodicity searchesof these surveys were 170 for Edwards et al. and 62 for Jacoby et al, giving a total of 230

70

Chapter 4. Transients in Archival Pulsar Surveys

Figure 4.1 A detection plot for a burst from J1825–33. Upper left: Time series fromeach of the 13 beams dedispersed at the DM of brightest detection; each field plots thepower versus time, normalised by the RMS of the time series. All fields plot the same
abscissa and ordinate range, and the beam (and sample in the time series) of brightest
detection is indicated by a red star. In the other beams, the sample of peak detection
is marked by a vertical dotted line, and the detected width of the pulse is marked by ahorizontal bar. Lower left: Time series for the maximum-SNR beam dedispersed at the
DM of brightest detection, stacked with the time series from one DM trial higher (“1up”),
one trial lower (“1dn”), and the DM = 0 time series. As in the upper left panel, theordinate gives normalised power (SNR), while the abscissa gives time since the beginningof the file (in seconds). Bottom right: A false colour image of the received power asa function of frequency (in MHz) versus time since the beginning of the file (in seconds),
revealing the frequency-dependent structure of the detected pulse in the beam of brightest
detection. Upper right: Signal-to-noise ratio as a function of DM trial and boxcar
filter size, and best-fit information about the detection (DM, SNR, pointing position of
the beam of brightest detection, and boxcar width “tscr”). Pulses with resolved complex
structure can show multiple peaks and other structure in their SNR vs. DM curve, thoughgenuinely dispersed pulses (and the curve above at the correct DM±5) generally obey thefunction given by Cordes & McLaughlin (2003).

4.2. Data Sample

71

Figure 4.2 Here we show a detection plot for a pulse wrongly identified by the software
as a candidate. The panel descriptions are as given for Figure 4.1. In this example, twosequential undispersed pulses with narrow-band structure generated power detected at
higher DM than zero, despite a smaller peak also at DM = 0 pc cm−3 . The narrow-bandstructure that generated the false identification is visible in the lower right panel. Note,however, due to its appearance in multiple beams, a multibeam filter less conservative
than ours would have eliminated this candidate.

72

Chapter 4. Transients in Archival Pulsar Surveys

independently detected pulsars.
All follow-up data presented here were taken with the new 2-bit digital filterbank
system currently installed at Parkes Radio Telescope that use CASPER tools and is known
as BPSR. Data with this system were taken with 1024 channels across 400 MHz centred
at 1381.8 MHz, and sampled at 64 µs. Filters blocking emission from a geosynchronous
satellite visible by the telescope render frequencies above 1520 MHz unusable.

4.3 Single Burst Search Process
The most significant difficulty in single pulse searches is allaying the millions of spurious
candidates which arise from local man-made and natural radio frequency interference, suchas lightning. Our search employs various stages of automatic radio frequency interference(RFI) elimination before candidates are manually assessed. We generally followed the stepsof Cordes & McLaughlin (2003), however with adjustments in the techniques used for RFI
rejection, matched filtering, and visual inspection. The main discriminant used in single
pulse searches is the fact that astrophysical transients exhibit a well-defined frequencydependent delay caused by the propagation through multiple ionised plasma screens in
the interstellar medium, visible as a time lag across the bandwidth of our instrument.
The dispersion measure, DM, measures the integrated line-of-sight column density of freeelectrons in units of pc cm−3 . Locally generated signals generally do not appear dispersed(DM ∼ 0), while genuine bursts have a decrease in measured significance as the trial

dispersion increases from the true dispersion of the source. Periodic sources will show
multiple detections at the same dispersion measure. The radio interference “forest” local
to any given astronomical observing site gives rise to the need for a set of thresholds
that remove broad, zero-dispersion signals. We therefore have increased confidence incandidates at higher dispersion.
Our detection thresholds were estimated by manual inspection of the data and trial
runs on a sub-sample of data to characterise the typical local RFI environment of theParkes Telescope. These tests indicated that events separated by at least 3.75 ms (30 data
bins at the native time resolution) could be considered independent, that signals peaking insignal-to-noise ratio (SNR) below DM = 5 were most likely RFI or statistically insignficant,and that setting a search threshold at six times the RMS noise of the dedispersed timeseries was sufficient to filter out the majority of candidates which were due to statisticalfluctuations or narrow band interference.
The search process followed several steps. At each trial DM, after dedispersion thefrequency channels were summed and a search for SNR≥ 6 pulses was performed of the4.3. Single Burst Search Process73

complete 4.4 minute observation. An increased sensitivity search for broader pulses wasthen performed, using a progressive box-car smoothing function ranging 21 , 22 , ..., 28 timesamples, corresponding to a maximum filter width of 32 ms. The 32 ms maximum width
was chosen to significantly decrease the ratio of local to astrophysical candidates (as determined by tests on a data subset), and to preserve our sensitivity to impulses of low
dispersion measure (the total delay in milliseconds across our band is roughly equivolent
in value to the dispersion measure of an impulse in units of pc cm−3 ). We have decreased
sensitivity to pulses of half-maximum width, w, outside the range 0.125 < w < 32 ms.
Additionally, the one-bit nature of the data made it difficult to discover pulses with width
less than a sample period, as we only had ≤ 96 functioning one-bit channels, and an RMS
that meant it was difficult to have much confidence before summing samples. We searchedin DM from 0 < DM < 600 pc cm−3 , which for the galactic latitudes of the survey issufficient to probe out to at least 50 kpc except in the regions a few tenths of a degreefrom l = 0, |b| = 5.
For each beam, a first-pass RFI rejection was performed by doing temporal coincidence matching for all trial dispersions and matched filter steps; in this way we were able
to reconstruct multi-plane information about singly-detected events. A further temporal coincidence search of all remaining events was done in the 13 beams, conservatively
rejecting all candidates which had detections coincident in more than nine. After thisstep, we have a full event archive in which are encoded the widest duration statistics for
detected beams, and information about the beam, DM, and smoothing filter of strongest
detection. Event archives which contained less than three events, and those which had apeak in signal-to-noise at less than DM = 5, were rejected. This step significantly reduced
the number of high dispersion candidates which were due purely to narrow-band RFI andsmall number statistics. The information in the remaining event archives were used tomake the plots used in manual candidate assessment.
The manual candidate ranking involved the viewing of each candidate using the tool
displayed in Figures 4.1 and 4.2. The panels contain information with which one canquickly discriminate a true burst (e. g. Fig. 4.1) from a false detection (e. g. Fig. 4.2). During candidate viewing, we simultaneously queried the ATNF pulsar catalogue for pulsars
within two degrees of the brightest beam, such that we were able to easily determine DM
and position incidence, and attribute detections to previously known pulsars. Non-RFI
detections not associated with known pulsars were considered new transient candidates.
Assuming that detections which show repeated pulsations are part of the neutronstar population, we fit a rotation period to each object which has emitted at least three

74

Chapter 4. Transients in Archival Pulsar Surveys

Figure 4.3 A galactic map of the pulsars originally detected in the data, plotted with the
new detections presented in this work.
J2000
Name

l
(deg)

b
(deg)

DM
(pc cm−3 )

d
(kpc)

Np /tobs(hr−1 )

P
(ms)

wi
(ms)

ws
(ms)

Smax
(mJy)

J0735–62†
J0923–31#
J0941–39† *J1129–53
J1226–32
J1534–46†
J1610–17#
J1647–36
J1654–23
J1753–38
J1753–12
J1825–33
J1850+15†
J2033+00†

274.7
259.7
267.8
290.8
296.9
330.0
355.6
347.1
357.9
352.3
14.6
0.3
46.7
46.0

-19.2
13.0
9.9
7.4
30.1
7.9
24.1
5.8
12.6
-6.5
6.7
-9.7
7.3
-22.2

19.4±7.872±2078.2±2.777.0±2.536.7±9.964.4±7.852.5±3.0
223.9±1.6
74.5±2.5
168.4±1.3
73.2±5.243.2±2.024.7±8.745.2±5.9

0.93
0.55
0.56
2.05
1.42
1.75
2.10
5.23
2.04
3.79
1.85
1.17
1.79
2.28

—
1.7
—
36.2
40.9*
—
13.6*
436.4*
40.9*
26.3
40.9*
14.4
—
—

4862.940(2)—586.77841838(3)1062.880(5)6193.1(5)364.835(9)—210.726(1)3986.48(5)621.000(5)405.454(1)1271.2(2)1383.965(3)5012.918(2)37.9
30(213)
105.6
19.1
60.7
25.5
5
9.9
0.52
4.8
18.8
16.5
31.8
95.2

14
30(213)
6
14
26
10
5
9
0.52
2.5
14
16
6
38

160
120
580
320
270
140
230
250
710
440
160
360
200
140

Table 4.1 Data for the 14 new sources detected by this search. Columns: (1) Name basedon J2000 coordinates (# signifies only one burst detected, while † denotes an object thatwas Fourier-detectable in the original pointings or in follow-up data; * see §4.4.1 for special
remarks on J0941–39); (2,3) galactic longitude (l) and latitude (b); these coordinates havean error of approximately ±15 arcminutes due to the poor positional localization of themultibeam receiver; (4) best-fit dispersion measure and error; (5) distance to the object
based on the DM and NE2001 galaxy electron density model (Cordes & Lazio 2002); (6)the pulsation rate based on all data taken on object position to date, where the pulsar
was not detected above a SNR of 5 in the folded data. An asterisk indicates an object for
which only the 4.4-minute detection pointings have been used to compute this number;(7) the best fit period to all detected pulses with error on the last digit in parentheses; (8)integrated pulse width measurement at 50% of maximum after the stacking of all detected
single pulses. For PSR J0923–31, we quote the narrow component width and the total
pulse width; (9) pulse width measurement at 50% of the peak of the brightest detected
pulse. PSR J0923–31 specified as in column 8; (10) peak flux density of brightest detected
single pulse.

4.4. Results

75

detectable pulses. Pulse periods were derived by first fitting periodicities by eye to thedetected pulses. The data were then broken into sub-integrations at the estimated period,and sub-integrations containing on-pulses were extracted and used to do an optimised
automatic search in period and DM space using the pdmp program distributed with thefreely-available PSRchive software suite1 . Where follow-up data was available and theobservations were of higher SNR, those data were used to compute the fits.

4.4 Results
Our blind search yielded the detection of several thousand single pulses with which we
could associate 91 pulsars, that were already previously detected through Fourier transform search techniques by Edwards et al. (2001) and Jacoby et al. (2009), indicating thatroughly 40% of the pulsars detected in their periodicity searches have one or more detectable pulses per 4.4 minute pointing. The pulsars we detected do not appear to haveany selective distribution in age or magnetic field strength, have periods ranging 0.089 to
8.5 seconds, and appear to have primarily populated the “island” of standard pulsars in
the period-period derivative diagram with periods between ∼0.1 and 3 seconds.

The new objects detected in this search include two sources which emitted only one(albeit very convincing) pulse over the time we have observed them, and 12 sources fromwhich multiple pulses were observed. All of these new sources are interpreted to be erratically emitting neutron stars. The best-fit parameterisations for each source are presented
in Table 4.1, and a more detailed description for each is given below. The implications of
the new detections are discussed in §4.5.

4.4.1

New Transient Sources

The properties of the pulsed sources presented here give no indication that they areassociated with a non-neutron star origin.

Below we describe the properties uniqueto each new object, and note the amount of follow-up time which has been dedicatedto each. Where emission was detectable in a Fourier search, the intermittency ratio(r = SNRSinglepulse /(SNR)Fourier , as given by D09) is provided. Note that after the determination of the DM from the brightest automatically-detected pulses, a manual inspection was done to identify weaker pulses (typically down to ∼4 times the timeseriesnoise). In cases where further pulses were detected, the weaker pulses were included in
the determination of the objects’ periods and event rates.
1

Available from the web on http://psrchive.sourceforge.net/

76

Chapter 4. Transients in Archival Pulsar Surveys

PSR J0735–62
This source emitted several pulses with SNR > 7 and many pulses below a SNR of 6 inthe discovery pointing, and after a period was determined from the brightest bursts, thesource was visible when the data were folded at the manually measured period with a SNR
of just below six. The source appears to experience severe scintillation of its signal due
to its low dispersion measure, and it is likely that because of this the six-minute tracks
on this source over 93 minutes of follow-up observations showed no detectable emission
in single pulses, however showed a weakly visible signal when the data were folded at
the nominal period and DM. Three-period averaged pulses from this pulsar are shown in
Figure 4.4(a). Despite its steady emission, the long period and narrow duty cycle prevent
it from being detected in standard Fourier searches of short duration (e.g. Deneva et al.
2009). The intermittency ratio of this object is r = 1.45.
PSR J0923–31
This source emitted only one pulse in the discovery observation, which appeared at ninetimes above the system noise and with complex profile structure with a bright, narrow
main component. This pulse is shown in Figure 4.5(a), and appears to have a faint,time-resolved, double peak. Subsequent pointings, totalling 30.9 minutes, revealed nofurther emission from this location. Further pulse detections are required to rule out
local interference as the source of the burst, particularly in light of the pulses presented
in Chapter 6. However, the appearance in only one beam of the multibeam receiver,frequency sweep that closely follows the cold plasma dispersion relation, and dissimilarity
to the Chap. 6 pulses indicate that the source is unlikely to be of the same origin, and istherefore most likely to be astrophysical. Based on a 10% duty cycle and a pulse width of
213 ms, if this object is a pulsar its estimated period should be > 2 seconds.
PSR J0941–39
This pulsar was discovered after the detection of five single pulses to which there was noclear periodicity. Follow-up observations of the source (totalling more than 3 hours spreadover approximately one-half year) later revealed extraordinary behaviour, sometimes appearing with sporadic RRAT-like emission at a rate of ∼90-110 per hour, and at othertimes emitting as a bright pulsar with strong pulse-to-pulse modulation and quasi-periodic
nulling (see Fig. 4.6). The pulsar has a triple-peaked profile and broad duty cycle, withcomplex sub-pulse drift, mode-changing, and longitude-dependent modulation when in an4.4. Results

77

(a)

(b)

(c)

(d)

Figure 4.4 Greyscale representations of time (or pulse index, y-axis) versus de-dispersed
pulse phase—“pulse stacks.” These four objects were alone in showing Fourier-detectableemission underlying their single bursts. PSR J0735–62 has been folded over three rotation
periods, PSR J1850+15 over six periods, and the others show single pulses.
“on” state (it is these behaviours which made single-pulse periodicity fitting difficult). A
timing program is underway on this pulsar at Parkes Telescope, and observations of theproperties of this object will be presented in detail in a forthcoming paper.

PSR J1129–53
This source was originally detected as two single bursts occurring at the same DM in
independent pointings separated by 14 arcminutes. Follow-up observations totalling 15.5
minutes at a position half-way between the original two detection positions (it is thisposition listed in Table 4.1) revealed 10 further pulses which were extracted and used tofit a period and DM. The individual pulses from this object exhibit a variety of pulse
shapes, with some pulses exhibiting evenly double-peaked emission.

78

Chapter 4. Transients in Archival Pulsar Surveys(a) PSR J0923–31(b) PSR J1610–17

Figure 4.5 Greyscale spectra (bottom panel) and dedispersed, frequency-integrated timeseries (top panel), showing the single pulses of the two objects from which no further
emission has yet been detected.
PSR J1226–32
Three pulses of SNR > 12 were detected from this object during the discovery pointing,and a best-fit to these pulses gave us the period and dispersion measure quoted in Table 4.1.
When folded at the fitted period, the rotations which did not contain pulses of SNR > 12
showed no detectable emission. We have not yet had the benefit of follow-up observations
for this object, however the three dispersed pulses emitted by the object show consistency
in both dispersion sweep and frequency-dependent modulation, indicative of the bursts’
propagation through the interstellar medium from a common origin.
PSR J1534–46
This object appears to be intrinsically modulated (and nulling) with single pulses appearing at 12-14 times the rotation period. The modulation power spectrum shows a peak at∼4 times the pulse period near the centre of the integrated pulse. The single-pulse emission from this pulsar is shown in Figure 4.4(b). The pulsar was detectable in a Fourier
search, giving r = 0.66. We have not performed follow-up observations of this source.
PSR J1610–17
Only one pulse was detected from this location, however there have been no follow-up
observations on the object. The original detection had a SNR of 7.4, and no other detections above SNR = 4 appeared in the 4.4 minute observation. The single detected pulse isshown in Figure 4.5(b), and like PSR J0923–31, shows a frequency sweep that agrees wellwith the expected frequency delay due to interstellar dispersion, and additionally does not4.4. Results

79

Figure 4.6 Two-minute (205 rotation) pulse stacks exhibiting PSR J0941–39’s “RRAT”(off) and “Pulsar” (on) phases (§4.4.1, 4.5.3). The integrated profile shape is shown in
the panel the the right. Tracks of the signal-to-noise ratio of the peak of each pulse with
SNR > 3 are plotted below the pulse stacks. The brightest peaks of the on- and off-phases
are comparable, indicating that the “off” state is not due to interstellar scintillation.

80

Chapter 4. Transients in Archival Pulsar Surveys

share the distinct properties of the Chapter 6 pulses. We suggest that although follow-up
is warranted, this pulse is likely to be of astrophysical, galactic origin.

PSR J1647–36
This pulsar appears to exhibit periodic nulling with bursts lasting on the order of 1-6 pulse
periods spaced by nulls lasting ∼27 seconds (130 rotations). With each period of “on”

activity, the bursts arrive at a phase offset by 5 − 10% from the previous burst episode; itis possible that the system is either strongly drifting or mode-changing, however follow-up
observations of this source are necessary to assess these hypotheses.

PSR J1654–23
The original detection showed 3 narrow (sub-0.5 ms), faint (SNR < 7) bursts which allowed
the fit reported in Table 4.1. The duty cycle is the smallest observed to date for a pulsar,
however due to the narrow pulses and weak emission it is possible that some pulses were
not visible above the noise. It is possible that the period reported for this pulsar could
easily be an integer multiple of the true period, the effect of which would be to broaden
the duty cycle by the same integer value. Follow-up observations of longer duration thanthe discovery pointing and at a faster sampling rate will reveal more completely the natureof this pulsar.

PSR J1753–38
The period and DM for this object were derived by extracting and fitting just the threebright pulses detected in the original pointing. Follow-up pointings (totalling 21 minutes)
revealed further pulses consistent with the flux, period, and DM of the original detections.

PSR J1753–12
The discovery pointing containing this object was badly affected by interference. Thisand the fact that only one very strong impulse was present made genuine pulse extractiondifficult, however three dispersed pulses from the object were identified and parameters
estimated. We have not yet made follow-up observations of this source, however cleaner
data may reveal more frequent low-level emission.

4.5. Discussion

81

PSR J1825–33
In the discovery pointing, this pulsar emitted seven bright, sequential pulses near the
beginning of the observation but was not visible thereafter, nor was any emission visible
in 25 minutes of follow-up observations. The profile structure of this pulsar has a double
peak (Figure 4.1 shows one pulse from this object).
PSR J1850+15
This pulsar’s discovery pointing contained strong periodic interference which probably
prevented its detection in the original Fourier search (r = 1.04; interference is visible
atop the fainter pulses of the object in Fig. 4.4(c)). The emission appears to be steady,with no evidence for clear intermittent behaviour, although there is strong pulse-to-pulse
amplitude modulation. We have not performed follow-up observations of this source.
PSR J2033+00
This source appears to be an intermittent pulsar with a high nulling fraction and is sometimes detectable in the Fourier domain. There is some evidence of sub-pulse drift; individual pulses rarely come from the same phase for more than a few rotations (see Figure
4.4(d)). The source was detectable in single pulses for the entirety of the 25 minute followup on the source, however became distinctly brighter (and weakly detectable in a Fourier
search; r = 1.86) in the final 14 minutes.

4.5 Discussion
4.5.1

Pulsar Transient Varieties and the Nature of RRAT Emission

Regular pulsars are themselves intermittent radio sources, showing detectable emission
only for the fraction of time that their beams sweep Earth. The individual pulses, however,are observed to show intrinsic pulse-to-pulse flux and shape differences, the origin of whichis yet unknown (though various theories exist). The variations observed in pulsars can be
broken down into two main types, being: pulse-to-pulse modulation, in which the object
emits continuously, however at an intrinsically modulated amplitude, and nulling, during
which the pulsar beam is somehow quenched, emitting no detectable radiation for a periodof time. In many pulsars, nulling appears to be a real phenomenon (with no underlying
emission when in null states), with nulls observed to occur from zero to 95% of a pulsar’s

82

Chapter 4. Transients in Archival Pulsar Surveys(a)Pulsar(b)
NullingPulsar(c)
ExtremeNullingPulsar(d)
RRAT

Figure 4.7 Windowing as a descriptor for the continuum of emission on/off cycles observedin pulsars, where the average timescale of the window function defines the behaviour
observed: (a) shows a regular pulsar, (b) is an example of a nulling pulsar, (c) an example
of what we term an “extreme nuller” (e.g. nulling fraction > 75% of pulses are nulled
and the on-window typically endures for a few times the rotation period), (d) an RRAT,
in which the typical on-window has a duration of less than the pulsar period. Note thatsome “on” windows will not result in detected pulses if the pulsar’s beam is not directed
towards Earth during the on-state.
observed rotations (Wang et al. 2007). Some pulsars appear to null for days-weeks, witha period derivative that is dependent upon the emission state (Kramer et al. 2006).
The range of nulling fractions observed in pulsars is easily described by a windowing
function, in which the number of sequential pulses observed at a given time is definedby an on-window of some length. In this description, regularly emitting pulsars havean indefinite on-window, while pulsars with increasing percentages of nulling have onwindows of increasingly short timescale, with extreme nullers (nulling fraction & 75%)
becoming more likely to be detected in single pulse searches than periodicity searches.
Various degrees of pulsar windowing are illustrated in Figure 4.7. It is clear that one
could incorporate the RRATs into the general pulsar population in this way, by justextending the non-nulling window to a time of length t < P , causing only one pulse to beobserved at a time. Implicitly, there should be a continuum in the distribution of nulling
fractions leading from constantly-on pulsars through nulling pulsars to the high nulling
fractions of RRATs. It is possibile that within this model, pulsars start off as continuous

4.5. Discussion

83

emitters, and gradually increase their nulling fraction until they become RRATs, which
ultimately evolve further to emit no radio emission.
The alternative view of RRAT emission, related to pulse-to-pulse modulation, wassuggested by Weltevrede et al. (2006b), who indicated that if PSR B0656+14 were observed
at several times its distance from Earth, the highly variable single pulses (emitting at up
to 420 times the average pulse flux) could emulate the single pulse behaviour observedby M06’s distant discoveries. The pulse-energy-distribution analysis of K09, however,makes an argument against this hypothesis (although the analysis was not done on “classic
RRATs” as we define them below). It appears obvious that there will ultimately be somefraction of RRATs that belong to this class, but deep integrations would be expected toshow underlying emission; the real issue, therefore, is in what fraction of RRATs belong
to this class, and if some do not, what are the properties of those sources that do not?
Whether an effect of modulation or nulling, the current competing theories of RRAT
emission can be broken into three possibilities, in which the RRATs represent:(1) a distinct class (intrinsic life-long single-pulsing)(2) faint pulsars with extreme pulse-to-pulse modulation(3) an evolutionary phase of pulsars, e.g. where the magnetic field degrades or the spin
period increases, leading to increased nulling fractions
Scenarios (1) and (3) intertwine similarly with questions about pulsars observed to null
(relating to whether nulling is an inherent phase of all pulsars or an effect of the local pulsar
environment). The clarification of RRATs’ nature not only has innate value, but the three
scenarios above have quite different implications for how the RRAT birthrate integrates
into that of pulsars, and how they contribute to the imbalance of core-collapse supernova rates and neutron star birthrates noted by Keane & Kramer (2008). For instance,in scenario (1) the latent RRAT population may actually exceed the pulsar population.
Scenarios (2) and (3), on the other hand, are in part solutions to the birthrate problem, inthat the birthrates contributed by RRATs are effectively absorbed into those of pulsars. If
RRATs are simply distant, highly modulated pulsars, the sources detected as RRATs are
simply abnormalities among the distant, otherwise undetectable pulsars sitting below theaverage luminosity limit of periodicity surveys; these distant and low-luminosity pulsars
are accounted for in pulsar birthrate analyses (e.g. Lorimer et al. 1993) with statisticalestimates, using luminosity-dependent scaling factors.
Although the original RRATs of M06 have a number of apparent properties which setthem apart from regular pulsars, as detailed in Sec. 4.5.3, we define the classic RRAT

84

Chapter 4. Transients in Archival Pulsar Surveys

as an object which emits only non-sequential single bursts with no otherwise detectableemission at the rotation period. When using the term nulling pulsar, we refer to an object
which emits for some fraction of its rotations, however the emission coming in bursts ofseveral sequential pulses. Finally, we refer to all objects which emit pulses for less than
∼25% of their rotations as extreme pulsar transients.

Below we discuss the properties of our new transients and compare them with the

RRATs of M06 and with the properties of the general pulsar population.

4.5.2

A Breakdown of Our Discoveries

What is immediately striking about the 14 objects detected in this search is the variety ofdormancy timescales, covering a broad range of the nulling and modulation phenomena
related above. For some objects, such as PSR J1850+15, the lack of previous detection by
Fourier search techniques can be explained by the presence in the data of intense periodic
RFI, which lowers sensitivity to fainter periodic sources. The pulsars PSR J0735–62 andPSR J1850+15 appear to simply be regular pulsars which were likely missed in previous
searches due to the modulation and long period of PSR J0735–62, and the presence ofinterference in the discovery pointing of PSR J1850+15.
The “classic RRATs” in our search include PSRs J1129–53, J1226–32, J1654–23,
J1753–38, and J1753–12. We also place the objects from which only one pulse was observed in this category; although we have not detected further events from the sources,
the pulse widths are typical of pulsars and the pulsation rates implied by our observations are consistent with the range of pulsation rates observed in the RRATs published by
M06, which ranged from one per four minutes to one per three hours. We note that thebright single pulses emitted by PSRs J1226–32 and J1753–38 are too bright and have apulsation rate too high to represent a long tail of a log-normal pulse energy distribution,
immediately indicating that scenario (2) above (extreme modulation) is not the case for
all RRATs. This is less clear in the case of PSR J1753–12, and we note that our sources
may represent a mix of highly modulated low-luminosity pulsars, and true RRATs.
Apart from PSR J0941–39, the remaining objects appear to be nulling pulsars withhigh nulling fractions. Only two of these objects were in an “on” state for a sufficient
fraction of observing time to be Fourier-detectable. Additionally, it is difficult to say
whether the source PSR J1825–33 is truly a nulling pulsar, or whether this source had
an event which briefly activated its outer gap, as we have since not detected any further
emission from the object.
PSR J0941–39 does not fit cleanly into any currently identified pulsar category, some-4.5. Discussion

85

times acting like a single-pulse-emitting RRAT, and sometimes like a nulling (though not
extreme-nulling!) pulsar. This object may play an important role in understanding RRAT
emission.

4.5.3

Exploration of RRATs and the Properties of Our New DiscoveriesThis search represents an important step in understanding extreme pulsar intermittents
in several ways. First, we have surveyed the largest area to date for such sources in aregion off the galactic plane, and added the discovery of 12 highly intermittent pulsars
to a small known population of such objects. Given that neutron stars are mainly bornfrom massive stars in the plane of the galaxy, the high galactic latitudes predispose us
to sample a population which has had a sufficiently high kick velocity and sufficient time
to reach its current position. Also, the automatic RFI rejection and additional candidate
visualisation techniques appear to have given us a higher sensitivity to low DM single
pulses, as seen by the fact that M06 discovered no sources at DM < 88 pc cm−3 while wehave discovered sources at a broad range of DM, with twelve of the fourteen detectionsat less than 88 pc cm−3 , and with previously known pulsars positively identified down
to DM = 3 pc cm−3 . Additionally, the latitude ranges of past RRAT searches (K06, D09,
K09) did not exceed 5 degrees. This increased DM sensitivity and high, complementary
latitude range makes our discoveries salient objects for assessing the inherent propertiesof nearby, RRAT-like bursting sources and how neutron stars with extreme intermittence
relate to the Galactic population of pulsars.
Throughout this discussion we analyse all of our sources which are sufficiently intermittent to not be detectable in a Fourier search—that is, the ten sources not plotted in
Figure 4.4. Where appropriate, we distinguish the analysis of “classic RRATs” and thatof “nulling pulsars.” For reference we also include the M06 RRATs and the four D09
objects which, as noted by their study, did not show detectable underlying regular emission. Additionally, we include the five objects which were not noted by K09 to exhibit
Fourier-detectable emission or clear nulling behaviour.
The sparse emission and dissimilarity of magnetic field strength to pulsars which emit
giant pulses on µs timescales (Cognard et al. 1996) are what most definitively led McLaughlin et al. to the conclusion that their sources were a new neutron star population, however
several other of the observed properties of the original 11 sources also distinguish themfrom the general pulsar population. This includes their low duty cycles, high single-pulse
luminosities, and their tendency to be distant (> 2 kpc, which may have partially been
due to a selection effect; this particular RRAT feature gives more weight to the claims of86

Chapter 4. Transients in Archival Pulsar Surveys

Weltevrede et al. 2006b that RRATs may simply be distant, highly modulated pulsars).
To further our understanding of the properties of RRATs, here we explore their various
distinguishing features in comparison to the extreme intermittents detected in our search.
The galactic distribution of our new objects (of the classic RRATs and of our 14
discoveries in general) agree well with the distribution of pulsars, concentrated more highly
towards the Galactic plane and the Galactic centre. Though not statistically rigourous
with a sample of fourteen objects, the galactic z-height distribution (z = d · sin(b)) of
our discoveries above z = 200 pc is well-fit with an exponential and a scale height of

300 pc, in good agreement with the galactic pulsar distribution. This agreement and thediscovery of RRATs in the high latitude surveys indicate that the RRATs are not a young
phenomenon confined to the galactic plane like anomalous X-ray pulsars (e.g. Durant
2006). The object of second largest z-height in our sample is the RRAT J1610–17 with ascale height of z = 0.86 kpc. With a birth at z = 0 pc and a proper motion outward fromthe galactic plane at 100 km s−1 , the implied age of this object is & 107 years.
Figure 4.8 plots the duty cycles of known pulsars and the objects analysed here. As
expected, the single pulse widths, ws , gleaned from our new objects lie primarily within
the optimal sensitivity range set by the box-car matched filters in our search, as describedin section 4.3.
The most notable feature of this diagram is that upon plotting our integrated pulse
width measurements (filled circles) with the single pulse width measurements (open circles), it becomes apparent that when using duty cycles measured from individual pulses(as were used in the calculations of M06, D09, and K09), the values measured for our
RRAT-like objects become systematically lower by a factor of ∼3, and up to a factor of

more than 10 for the extreme case of J0941–39. There may be several causes of this; forpulsars with nulling, pulse drifts and mode changes are often observed, which can work to
broaden the integrated pulsar profile (illustrated well by PSR J2033+00’s pulses in Fig.

4.4(d) above). Multi-component integrated profiles, made up of a superposition of otherwise narrow pulses emitted from a range in latitudes, can also broaden an object’s duty
cycle between single and integrated pulse measurements (as has occurred in the extreme
case of PSR J0941–39 with its triple-peaked profile). It is clear that were an integrated
pulse width measured for the sources of M06, D09, and K09, the duty cycle distribution of
RRATs would more closely resemble that of the general pulsar population, instead of appearing to sit characteristically lower than pulsars (the composite profiles for two RRATs
shown in McLaughlin et al. (2009) verify this suggestion). As the RRAT and pulsar duty
cycle distributions are similar, it is probable that the beaming fraction of such objects will

4.5. Discussion

87

1

Duty cycle

0.1

0.01

0.001

0.0001All known Pulsars (P>0.05 s)
D09
M06
K09
This work (ws)This work (wi)
100

1000

10000

Period (ms)Figure 4.8 The duty cycle versus period for various detections (as detailed in §4.5.2).
Among the sources presented in this paper, pulsars are highlighted in blue, RRATs inblack, nulling pulsars in grey, and PSR J0941–39 is represented by the red symbols. The
duty cycles of our detections are shown in two ways: using the integrated pulse width(measured after all available on-pulses were stacked at the pulsar period) and using thewidth of the brightest single pulse. M06, D09, and K09 reported only single-pulse widths.
The dotted lines represent traces of w = 0.125 ms and 32 ms, beyond which our searchsensitivity to individual pulses drops significantly.

Chapter 4. Transients in Archival Pulsar Surveys

Smaxd2 (mJy kpc2)

88
10

5

10

4

103

10

This work
D09
M06
K09
Smax = 66 mJy
Smax = 704 mJy

2

101

0

1

2

3
4
5
Distance (kpc)

6

7

8

Figure 4.9 Dependence of luminosity on the distance of the sources. The lower curve
represents our single pulse flux sensitivity limit for a pulse width of 32 ms, while theupper curve shows a representative limit above which a highly modulated pulsar would
become detectable in a Fourier search, as detailed in §4.5.3
likewise be akin to that of pulsars.
Next we examine the distribution of the single-pulse luminosities, as shown in Figure
4.9. The points cover a broad range in both distance and luminosity, appearing to show
a downturn with proximity to the observer. The lower single-pulse sensitivity limit of oursearch obeying L ∝ d2 is plotted (at 66 mJy for our 32 ms filter). In light of the possi-

bility that scenario (2) (modulation) is true, we consider the following: Fourier transformsearches for SNR ≥ 8 pulses as done by Edwards et al. (2001) over the 4.4 minute survey

pointings have a sensitivity limit to pulsars (given a typical duty cycle of 5%) with anaverage single-pulse peak flux of hSpeak i ≥ 3.3 mJy. The prominence and statistics of

extremely modulated pulsars (like PSR B0656+14) are not yet well characterised, however the study of Weltevrede et al. (2006a) of 187 pulsars at 1 GHz revealed that typicalmodulation rarely exceeds ten times the average pulse energy (with pulses exceeding thisrange typically lasting nanoseconds, and called “giant pulses”, Cairns e.g. 2004). Theirfollow-up study of PSR B0656+14, as previously noted, revealed infrequent single pulse
emission peaking up to 420 times the average single pulse intensity. Here we consider thatsuch extreme sources will be relatively uncommon, and take a population of hypotheticalsources bursting at half the maximum Smax /hSpeak i ratio of B0656+14, that is, a bursting population with typical Smax = 210hSpeak i. If such a population is contained in ourdata, sources above the limit of hSpeak i ≥ 3.35 mJy will have bursts at Smax ≃ 700 mJy.
Sources brighter than this limit will be detectable in Fourier searches and be tagged as4.5. Discussion

89
0.45

Pulsar population (P > 50ms)
RRATs

0.4

Normalised value

0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0

2

4

6

8

10

12

Period (s)Figure 4.10 Normalised period distributions for pulsars and the single-burst emitting
RRATs of M06, D09, K09, and this survey.
pulsars, however below this limit the periodic emission will have been missed, with only
the brightest bursts detectable in single pulse searches. This limit is plotted in Fig. 4.9,
and interestingly, appears to correspond to the upper envelope of the maximum source
luminosities found by our single pulse search.
The natural interpretation of this is that the nearby, highest-luminosity sources exhibiting erratic emission had bright enough low-level emission that instead of being detected
as nulling pulsars or RRATs, they were discovered in periodicity searches and taken into
the population as pulsars (albeit modulated or nulling ones). When a similar test is done
for the RRATs found by M06 (whose longer, 35.5 minute pointings have higher sensitivity
to periodic signals and a higher likelihood of containing the rarer brightest outbursts ofa highly modulated pulsar; a periodicity search of that survey is described in Manchester
et al. 2001), two sources, PSRs J1317–5759 and J1819–1458, still lie significantly above thelimit of the Fourier analysis. In the framework of our interpretation, these two sources are
therefore either examples of extreme sources among modulated pulsars (however nearby
equivalents among the known pulsar population, apart from B0656+14, have yet to berecognised), or alternatively they are simply examples of genuine RRATs, where the intrinsic pulsar emission is quite strong, however usually invisible because of a very highnulling fraction.
Among the four of our RRATs for which we could measure periods, although very
statistically poor, the previously noted RRAT feature—that they have a tendency tolonger periods than pulsars—is observed in our new objects, where 50% have periods
longer than one second. Plotted in Figure 4.10 are the normalised distributions for our90

Chapter 4. Transients in Archival Pulsar Surveys

and the published “classic RRAT” sources as compared with the known pulsar population
for pulsars with P > 50 ms (to restrict the analysis to non-recycled pulsars). When a
Komolgorov-Smirnov test is run on the un-normalised distributions, it results in a highprobability, P = 0.999985 (> 4σ), that the distributions are not drawn from the same
parent population. Few pulsars have periods greater than P = 3 s and because sources
such as anomalous X-ray pulsars/magnetars/soft gamma-ray repeaters all have periods
greater than a few seconds, it is tempting to draw an association between such sources
and RRATs; the recent release of timing data on six RRATs by McLaughlin et al. (2009)
show a further similarity of RRATs to these other neutron star types in their placementin the period-period derivative diagram, and a similar result in periodicity distributiondifferences. However, the broad distribution of source periods and the presence of 7
RRATs with periods below one second suggests that a more basic factor in the population,for instance age, which can create the observed tendency to longer periods. For the
RRATs which have had timing analysis done, all appear to have ages of a few times 105−6
years. However, the statistical analysis of McLaughlin et al. (2009), which comparedvarious derived RRAT and pulsar quantities, does not indicate a strong difference in the
distribution of RRAT and pulsar ages, though does indicate a strong probability that the
distribution of RRAT and pulsar magnetic field strengths are significantly different. Thereappears to be no relation between RRAT period and distance from the galactic plane (aproxy, albeit a loose one, for pulsar age).
The source PSR J0941–39 contributes a peculiar voice to this discussion, appearing
for hours, to possibly weeks at a time like a classic RRAT—erratically emitting single,
non-sequential pulses at a relatively constant rate—while at other times appearing as anintense (∼ 10 mJy) pulsar with a < 10% nulling fraction, sub-pulse drift that is strongly
latitude dependent, as well as deep latitude-dependent modulation. The object appears
superficially to tie a close link between nulling phenomena and RRATs, possibly signifying
a pulsar which is in some way transitioning from a pulsar mode to an RRAT mode. If
this is the case, the source provides a link both between RRATs and pulsars as well asa link to older, non-radio-emitting neutron stars—i.e. that the RRAT phase represents a
degradation of the radio emission mechanism before advancing beyond the radio pulsar
death line. As a timing solution is acquired for this pulsar, it will be interesting to note
whether it is of an advanced age. It is rather surprising how luminous this pulsar is, andwhat a small nulling fraction it possesses when in its “pulsar” state. It is important tonote that the site of the RRAT emission is coincident with the three peaks of the profile
when the pulsar is on. Thus, the RRAT emission is from the same site as the normal

4.6. Summary91

pulsar emission, again implying similar beaming fractions for the two states.
The properties of the new transients found in this search, and particularly of the five
new RRATs, give various indicators that RRAT behaviour is well described by either orboth of scenarios (2) or (3) above. The idea that observed RRAT behaviour may be anobserved consequence of extreme (> 99%) nulling is supported by the high nulling fractionof the objects tagged as “nulling pulsars” above; while previously the “record” nulling fraction was held by PSR B1713–40 at 95% (Wang et al. 2007), PSRs J1825–33, J1647–36, andJ1534–46 all have implied nulling fractions of more than 97%, telling us that these objects
may imply a “bridging population” of high-nulling-fraction pulsars, while objects withhigher nulling fractions (smaller and less frequent on-windows) exhibit RRAT behaviour.
A full statistical nulling fraction analysis of the Edwards et al. (2001) and Jacoby et al.
(2009) pulsars may indicate that the nulling fraction of the RRATs discovered in thissearch fall naturally in the tail of pulsars with significant nulling fractions shown to exist
by Wang et al. (2007) in other surveys. This proposition, along with the lack of RRAT features that are physically distinct from pulsars (except their period distribution)—including
their galactic and duty cycle distribution—point to a strong relationship to the “normal”
radio pulsar population. The story is complicated and strengthened by the apparent
mode-switching of PSR J0941–39, which lends significant support for scenario (3) in that
it is clear that the object in its RRAT mode is not simply a modulated pulsar, and it is
likewise clear that the object is both RRAT and pulsar in nature, signifying a transition
between the two populations and more speculatively, implying that nulling pulsars may
undergo stepped increases in their nulling fraction with age. The analyses of Wang et al.
(2007) indicate no correlation between nulling fraction and characteristic pulsar age, however the number of pulsars exhibiting nulls appears to increase with proximity to the radio
death line (see their Fig. 8). If all RRATs are relics of nulling pulsars, the broadening ofintegrated pulse measurements supports the fact that despite their sparse emission, theemitted pulses may still be exhibiting drifting or mode-changing phenomena.

4.6 Summary
We have re-examined the intermediate galactic latitude surveys of Edwards et al. (2001)
and Jacoby et al. (2009) for single pulses of radio emission of duration 0.125 < w <32 milliseconds. This search revealed 14 intermittent sources which appear to be neutronstars. Periods ranging 0.2 to 6.2 s were measured for 12 of these sources. While only four ofthe objects could be detected retrospectively by Fourier search techniques, the remaining
10 exhibited intermittent emission behaviour akin to both extreme nulling pulsars and92

Chapter 4. Transients in Archival Pulsar Surveys

the classic RRATs discovered by McLaughlin et al. (2006). The sensitivity of our searchtechniques to bursts of lower dispersion measure allowed us to find objects which werecloser than those detected by previous searches.
The general properties of the new sources appear to paint details of the extreme intermittent and RRAT population that concur with having similar emission modes andgalactic distribution to pulsars, despite being significantly less emissive than the general
population. It was found that the duty cycle distribution of our detections mimics that ofpulsars. These similarities, and the discovery in this search of pulsars with extremely highnulling fractions, lead us to suggest that RRATs may represent an extreme type of nulling
pulsar whose window of on-activity is shorter than the rotation period of the object, andthat the sources observed to exhibit RRAT behaviour are a natural extension of the tailof pulsars observed with high nulling fractions. Furthermore, PSR J0941–39 appears tooscillate between a clear “nulling pulsar” state and a more intermittent “RRAT” mode.
This source may represent the first detection of an object in transition from a bright pulsar
with an average nulling fraction to a single-bursting source, closely tying nulling pulsars
to RRATs and indicating a possible evolutionary progression. Further studies and timing
are warranted and upcoming on this object.
We conclude by noting that identifying RRATs is not a simple task, as they may
represent a mixed population of intrinsically single-pulsing pulsars and highly modulated
pulsars whose brightest single pulses singly exceed the system noise. Modulated pulsars
appearing as RRATs are expected to be few, as among the local population of pulsars,
PSR B0656+14 is currently the only pulsar known to be as modulated as the population
required for RRAT emission. Nearby pulsars which are part of such a population may inthe future be identified by pulse energy distribution studies of nearby highly modulated
pulsars, using long observations which have a chance of detecting the rarer brightest
outbursts from such objects. Statistical studies of these pulsars may help separate “true”
RRATs from simply highly modulated pulsars.
To discern the nature of RRATs, other extreme pulsar intermittents, and highly modulated objects, it will be critical in coming years to both identify more such sources, andto continue timing analyses on the known sources to determine their relationship to otherneutron star populations. Of additional interest are the statistics of intermittent phenomena in extreme nulling/modulated pulsars among the known pulsar population, theanalysis of which may reveal further insight into the relationship between regular pulsars
and the vast variety of intermittent behaviours observed in these equivocal neutron stars.

5Single Pulse Analysis in the The High Time
Resolution Universe Pulsar and Fast-Transient
SurveyOmnia mutantur, nos et mutamur in illis!
—M. Borbonius
In this chapter we describe the search for transient radio signals in the new High Time
Resolution Universe (HTRU) Survey. We present here: 1) the HTRU Survey’s single-pulse
search and follow-up strategy, and the software designed to perform the core single pulse
search, 2) a determination of the HTRU Survey’s sensitivity to single pulses, and 3) the
13 new neutron star discoveries made thus far with the single pulse pipeline. We returnto the “windowing” description of the previous chapter to explore RRATs and our new
detections in terms of emissivity timescales, and demonstrate the intermittent populations
that the HTRU Survey can explore, and note that the survey may discover more than 200
new sparsely emitting neutron stars by its completion.

5.1 Introduction
The High Time Resolution Universe Pulsar and Fast Transients (HTRU) Survey is the firstall-southern-sky survey for pulsars and fast transients using an all-digital backend, covering declinations δ < 10◦ (Keith et al. 2010; hereafter HTRU Paper 1). The survey employs
a new digital backend, the “Berkeley-Parkes-Swinburne Recorder” (hereafter BPSR), thathas been installed at Parkes Telescope for the 20 cm multibeam receiver (Staveley-Smithet al. 1996). BPSR allows improved digitisation levels, and frequency and time resolution
over the previous analogue instrument. This affords the HTRU survey unprecedented93

94

Chapter 5. Transients in the High Time Resolution Universe Survey

sensitivity to short-duration and highly dispersed single impulses of radio emission in thesouthern sky. This chapter describes the techniques used in the survey to search for single
pulses. Because this survey required concurrent searches using periodicity and single-pulse
techniques, it affords the opportunity to explore efficient modes of search operation, attempting to perform these techniques in parallel with a minimum of duplicated computing
effort.
The targets of our search are similar to those described in Sections 1.4.3, 1.4.4, and4.1, and include RRATs, giant pulses from pulsars, radio bursts of extragalactic origin,terrestrial impulses (e. g. Chap. 6) and other unanticipated, yet undetected sources ofcelestial transient radio emission.
The HTRU intermediate (galactic longitudes −120◦ < l < 30◦ , latitudes |b| < 15◦ )and high (covering declinations δ < +10◦ not covered by intermediate pointings) latitude

surveys share common areas of sky coverage to previous surveys which employed theanalogue filterbank installed on the Parkes Multibeam receiver.1 The “Parkes Multibeam
Survey” (Manchester et al. 2001) covered |b| < 5◦ with 35-minute pointings, and wassearched for single pulses by McLaughlin et al. (2006) and Keane et al. (2010), revealing

the bulk of the currently known RRAT population. Hereafter we refer to these searches
collectively as the PKSMB searches. At higher galactic latitudes, a single pulse search was
performed on the Jacoby et al. (2009) and Edwards et al. (2001) surveys, as presented in
Chapter 4. We refer to that search throughout this chapter as BSB, in reference to therefereed publication in which the search was also described (Burke-Spolaor & Bailes 2010).
Despite the common sky coverage and high success rate of the BSB and PKSMB searches,
we nevertheless expect to discover any source of impulsive radio emission in these areas;some pulsing radio sources may not have been previously detected due to the source’s low
event rate, insufficient sensitivity in previous surveys, or the improved dynamic range of
the BPSR observing system over the previous analogue filterbank system installed on the√Parkes multibeam receiver, whose dynamic range was hurt by N -statistics (explored in
more detail in §5.2.2 below).

In this chapter we present the search methods, sensitivity, and current status of

the HTRU Survey single pulse search (§5.2), and report on initial discoveries for theintermediate- and high-latitude surveys (§5.3). In §5.4, we determine why our discoveries
were not seen in the previous surveys of overlapping regions, present our new discoveriesin terms of the windowing description of pulsar intermittence presented in Chap. 4, andmake predictions for the sparsely-emitting radio neutron stars that the full HTRU survey
1

Note: although no processing has yet commenced for the HTRU low-latitude survey, that survey will
cover areas |b| < 3.5◦ and −80◦ < l < 30◦ , with 70 minutes per pointing.

5.2. The HTRU Survey’s Single-pulse (SP) Search

95

will uncover.

5.2 The HTRU Survey’s Single-pulse (SP) Search
5.2.1

Data Analysis PipelineThus far all processing and analysis has been carried out using the “Green Machine”
supercomputer at the Swinburne Centre for Astrophysics and Supercomputing. This supercomputer is made up of ∼150 dual quad-core CPU nodes that accommodate large-scaleparallel and distributed processing. The pipeline employs several standard search techniques (e. g. dedispersion and boxcar matched filtering), and several novel techniques(independent-event identification and inspection, multi-beam coincidence matching, distributed computing). The basic methodology of the SP search is based on that describedin Chap. 4.
There are two main bottlenecks in the HTRU Survey’s single-pulse search. The first is
the computational load demanded by the dedispersion of our data, and the second is thenumber of candidates which need to be manually assessed. The single-pulse data reduction
pipeline design includes schema that aim to abate both of these. First, we minimise the netcomputational time of the periodicity and SP searches by integrating the SP search into thehitrun processing pipeline introduced in HTRU Paper 1. By doing this, the interference
excision and dedispersion described in HTRU Paper 1 is done only once, producing eachdata stream’s results simultaneously for use in the single-pulse search and the periodicity
search. As described below, the time series search for single pulses is performed while
each time series is still in the computer’s “random access memory”, and for each 9-minutedata stream adds <2.5 minutes of processing time onto the total ∼3-hour timescale for thehitrun Fourier-based pipeline. We stress that the added computational load of including
a single pulse search to a traditional Fourier search is negligible.
We address the issue of large candidate numbers by identifying independent events
across multiple dispersion, boxcar filter trials, and beams, and by performing a simple
threshold-based interference zapping. The simultaneous processing and viewing of datafrom multiple beams decreases the occurrence of candidates caused by low-level, local
interference and greatly increases the speed at which the manual assessment of a candidate
can be made. However, due to limited computational resources, we cannot store all 13
beams on one node, requiring the use of a monitor to track and collect beams from eachpointing as they are processed.
The single-beam SP search and the multibeam data monitor are described below.

96

For all DM trials

Filterbank data
RFI pre-removal
Time, channel mask

Dedisperse

Periodicity
searchWrite
timeseries

Remove baseline
Store indatabase:
Yes
SNR,
Significant events?
start time,Nb= 21,22,
width, DM,… 29
N-sample boxcar filter
boxcar size

Coincidence match(all DM, boxcar trials)

Yes

Reconstructionof events

Do events have
< 3 members?
No

Yes Do event signals No
peak at DM < 1?

All available beams in pointing

CANDIDATEEVENTSREJECT

Write singleevent files

Spurious/RFI
detection

Files list members of
independent events

Pointing event
database

Multibeam
monitorCoincidence match(all available beams)

Collect event
data segments

Reconstructionof events(all available beams)

Candidate Plots
Events simultaneousin > 9 beams?
Yes

No

REJECTCANDIDATE

Likely local
signalFor manual
inspection

MANUAL ASSESSMENT

Chapter 5. Transients in the High Time Resolution Universe Survey

Figure 5.1 A diagrammatic representation of the HTRU Survey’s single-pulse pipeline.
Each of up to 13 “single beam” processes associated with one multibeam pointing are run
simultaneously, distributed onto several compute nodes and producing single-pulse andperiodicity search results for that beam. The multibeam monitor tracks the status of theindividual beams, collecting single-pulse search results, performing interference excision
based on multiple-beam occurrence, and producing single-event diagnostic plots for each
event in the pointing not tagged as interference.

Singlebeam

5.2. The HTRU Survey’s Single-pulse (SP) SearchParameter
Avg. pointing duration (s)Total time processed
Total area processed†
Fraction of total survey
Center frequency (MHz)Total bandwidth (∆f , MHz)
Typical usable Nchan
Sampling time (tsamp , µs)

97

Latitude region
Inter.
High
540
270
3490 h140 h
27671 deg
591 deg2
23.5%
0.39%
1352
340
870
64

Table 5.1 HTRU Survey observing parameters and an indication of the processed portionof the survey. Full survey parameters are given in HTRU Paper 1. Notes: † The typicalsingle-beam field of view is calculated within the average half-maximum beam width (≃0.044 deg2 ).
Figure 5.1 gives a schematic diagram for the pipeline, showing one beam and the multibeam
monitor.
Single-beam Processing
For each observation, a multibeam monitor is initiated and the available beams for thatpointing are submitted in sequence for processing on the supercomputer, each processedindependently using the hitrun pipeline. For this pipeline we begin with 2-bit filterbank
data with the sampling properties summarised in Table 5.1. Pre-analysis radio frequency
interference excision is done first, as detailed in HTRU Paper 1. In brief, this excision
creates and uses both a frequency and a time mask that flags periodic-interference-affectedchannels and time samples with a DM = 0 pc cm−3 signal above a signal-to-noise ratio(SNR) of more than 5. Flagged time samples are replaced in the filterbank file with noisedrawn from adjacent unflagged samples, while offending frequency channels are blanked.
While the time-domain interference excision weakens the SNR of bright signals of DM <0.12(w/64 µs) pc cm−3 , where w is the pulse’s width, it allows us to produce and inspect
candidates down to DM ∼1 pc cm−3 while maintaining a manageable false detection rate

at the manual inspection stage.

We perform dedispersion at 1196 trial DMs over the range 0 to 1000 pc cm−3 using
the dedisperse all program.2,3 This software is able to perform both dedispersion and
2This software and the C++ gtools library, which is drawn from by dedisperse all for single-pulse
functionality and contains the SP search algorithms, candidate matching functions, and SP candidate typeclasses, are available from http://www.github.com/swinlegion
3
Note that although the choice of 1000 pc cm−3 as a maximum was somewhat arbitrary, at most galactic
positions it allows the search of a large extragalactic volume as well as galactic-origin emission.

98

Chapter 5. Transients in the High Time Resolution Universe Survey

(a)

(b)

(c)

Figure 5.2 (a) A single-beam “summary” showing a beam near PSR J1129–53. 13 suchplots are made for each pointing. Events are coloured by auto-identified event type, as
determined by the process described in §5.2.1. Events flagged as zero-DM interference
are black, those gaussian peaks are red, and candidate events are blue. The upper centre
panel shows SNR against DM, and the left panel shows a histogram of the number ofcandidate (blue) and all (black) event members in the beam. The bottom panel shows
the DM vs. time, with the SNR encoded by size of the plot point. Here, six candidate
pulses have been correctly discriminated from noise and interference in the observation.
(b,c) Single-event plots corresponding to the second and third pulses detected from PSR
J1129–53 in panel a. Panels are as in Fig. 4.1. Deviations in the SNR vs. DM curves in (b)and (c) from the predicted Cordes & McLaughlin (2003) curves are the combined resultof noise, and both time- and frequency-dependent pulse structure.

5.2. The HTRU Survey’s Single-pulse (SP) Search99

a complete single-beam search for single pulses across a range of dispersion measures(including all components of Fig. 5.1 within the “single beam” panel after the interference
pre-removal step). The efficient dedispersion aspect of this code is described in detailby HTRU Paper 1. After reading the input filterbank data into computer memory, theprogram forms time series for each DM trial. As the time series data stream at each DM
trial is produced, it is used to perform a single pulse search with parameters as in BSB. Our
input search parameters differ only in that the boxcar filter used in our search ranges in size
Nb = 1 to 512 samples, and that events separated by more than 10 samples (640 µs) wereconsidered independent. All significant (as defined by our pre-set SNR threshold, mt > 6,
detailed below) events in each boxcar and DM trial are recorded and stored in a database.
At the completion of dedispersion of all trial DMs, dedisperse all performs a temporalcoincidence matching (similar to the “friends-of-friends” method developed by Huchra &Geller (1982) and used by Deneva et al. (2009) and BSB to identify time-coincident event
clusters). Each event is defined by the parameters (DM, boxcar filter, width, time) at
which the signal-to-noise is found to be the greatest, and consists of members at other
DM and boxcar trials found to be coincident.
The presence of man-made interference (i. e. radar communications, satellite and aircraft transmissions, on-site hardware, and a number of other sources) during observations
causes the noise in our time series data to follow a non-Gaussian distribution. These signals are typically either not dispersed, or dispersed to a level that is undetectable in ourdata, and therefore the zero-DM timeseries interference mitigation performed before processing removes the bulk of these signals. After interference pre-removal, however, some
low-level inteference signals remain, and we employ pre-set search thresholds to balance
the false detection rate. For most data, the interference pre-removal allows time series attrials above DM = 1.5 pc cm−3 , in the absence of our target astrophysical signals, to beroughly Gaussian-distributed. At lower dispersion trials, the false detection rate becomes
unmanageable, and we therefore set a dispersion-based threshold to reject all events foundwith a peak SNR at DM < 1.5 pc cm−3 . For each 9-minute intermediate latitude beam,
we produce 1196 dedispersed time series of Ns ≃ 8450000 samples. Our total number ofPsearched data points for each file consists therefore of 1196 × 9i=0 (Ns /2i ) ≃ 2.02 × 1010 .

In Gaussian-distributed data, at our SNR threshold of mt = 6 we would expect approxi-

mately 50 independent random noise detections per file. We attempt to filter these events
by automatically rejecting events which have less than three associated members. This isan effective method of Gaussian event rejection; noise peaks at a SNR of 12 or below are
not likely to exhibit SNR > 6 in more than two trials, assuming the peak’s signal between

100

Chapter 5. Transients in the High Time Resolution Universe Survey

DM steps and boxcar trials drops by at least√

2. Furthermore, the number of statis-

tically random events above SNR = 12 is negligible, even when considering analysis of
the full HTRU intermediate latitude survey. This strategy weakly impacts our sensitivity
to events of w = Nb tsamp that have SNR . 12. However, in concert these event-matchbased candidate rejection typically reduces the number of candidates by a further factorof ∼100–1000, and much more for data badly affected by interference. In Fig. 5.2, weshow the two visual inspection plots used for manual candidate descrimination. For theobservation displayed, the telescope was pointed at PSR J1129–53, a known RRAT (BSB).
The six pulses emitted by the pulsar in this pointing were correctly discriminated by thesoftware from zero-dispersion interference and from spurious peaks in the data.
All non-interference events are then written to disk by dedisperse all, producing
one ASCII file per event, in which all members of that event are listed. At this stage, thesingle-pulse events for the beam are gathered by the multibeam data monitor associatedwith the beam’s pointing. The time series are written to disk, stored for use in the
periodicity search and for later access by the multibeam data monitor.

Multibeam Data Monitor
As single-pulse events are produced for all the beams associated with one pointing, themultibeam monitor for that pointing collects the events and performs a temporal coincidence match of identical form to that done by dedisperse all, however it is performed
for events from different beams.
The Parkes 20cm multibeam receiving system was described in §4.2, but is reviewed

in brief here. The system allows the simultaneous observation of 13 positions on thesky with approximately 30 arcminutes between beam positions, and each beam has asensitivity fall-off on scales of <30 arcminutes from its pointing centre (Staveley-Smithet al. 1996). Pointlike radio sources boresight to the telescope pointing direction will
therefore typically be detected in a maximum of three beams of the receiver. Particularly
luminous objects like the Vela Pulsar, which emits single pulses up to ∼60 Jy, may detected

in up to ∼7 beams when ideally positioned in the multibeam field. Typically, signals of

sufficient brightness to appear in all beams at similar intensity, i. e. through a far sidelobeof the telescope, are generated by terrestrial or near-Earth sources (such as satellites oraircraft). Therefore, as in BSB, we do not inspect candidates which were detected in
more than nine of the thirteen beams. This filter decreases our sensitivity to the type of
impulses described in Chap. 6, however there is significant benefit gained through decreasein the false detection rate per pointing. A targeted search for the objects of Chapter 6

5.2. The HTRU Survey’s Single-pulse (SP) Search101

is planned for the near future. All events occurring in nine beams or less are imaged as
described in BSB, collecting the relevant filterbank and time series data from the original
on-disk location of the single beams. When both single-pulse and pulsar searches havebeen completed, the data is freed for removal from disk.
Finally, a pointing’s result plots (i. e. those for individual beams and events as inFig. 5.2) are manually assessed to determine whether they contain a detection of interest.
The “beam summary” plot has superiour sensitivity to objects emitting multiple pulses
with signals at or just exceeding the detection threshold, while the single-event plots allow auser swift discrimination between interesting candidates and falsely-identified interferenceor noise. During the manual inspection stage, results are also compared with the most
up-to-date version of the Australia Telescope National Facility Pulsar Catalogue originallypublished by Manchester et al. (2005).

5.2.2

Search Sensitivity

The sky coverage of the HTRU survey affords improved sensitivity in overlapping regions
for previous blind single-pulse searches of the southern sky. Particularly for the HTRU
intermediate latitude survey, we have direct overlap with the areas searched by BSB and
PKSMB. Here we calculate our sensitivity to transient events and make a comparison tothese surveys.
Instrumental and interstellar pulse broadening serves to weaken the SNR of the pulse
from its signal at the pulse’s intrinsic width, wi . All instrumental broadening effects are inprinciple avoidable, therefore the back-end design of any observing system aims to reachthe scatter-broadening limit, as this is the theoretical best that a pulsar/transients survey
can achieve. As noted in HTRU Paper 1, for a large range in dispersion measures the
improved time and frequency resolutions of the HTRU survey causes pulse broadening
from interstellar propagation to dominate over intrachannel dispersion-measure broadening (tch ). Using the empirical DM-scatter broadening relation given by Bhat et al. (2004),
we note that scatter broadening (tscat ) typically dominates over hardware-induced smearing above a DM of ∼230 pc cm−3 for the HTRU survey. To calculate our sensitivity tosingle pulses, we take these pulse-scattering and broadening effects into account.

Following Deneva et al. (2009), the observed pulse width depends on wi and various
pulse-broadening effects by w = (wi2 +t2ch +t2scat +t2samp )0.5 . Note that our step size between
DM trials is chosen such that the broadening due to an error in DM is small compared toother broadening effects. Our sensitivity limit for the intrinsic peak flux density of single

102

Chapter 5. Transients in the High Time Resolution Universe Survey

100

HTRU
BSB
64 µs

10

Sint (Jy)250 µs

1

2 ms

0.1
30 ms

0.01
0

100

200

300

400

500

600

700

800

3

Dispersion measure (pc/cm )Figure 5.3 The HTRU Survey single pulse search sensitivity to pulses of various intrinsic width,
compared to the performance of the Parkes 20cm analogue filterbank used by BSB (note: PKSMB
had tsamp = 250 µs whereas BSB had tsamp = 125 µs; thus the PKSMB curve lies slighly above
BSB’s). BPSR’s higher frequency resolution most markedly improves our sensitivity to narrowpulses at low DMs; above DM ≃450 pc cm−3 , the Bhat et al. (2004) model for interstellar scattering
dominates the instrumental broadening for both surveys. Because of our limited search range inboxcar matched filter sizes, there is a sharp decrease in our sensitivity for pulses of duration> 32 ms.

5.2. The HTRU Survey’s Single-pulse (SP) Search

103

pulses with observed duration w = Nb · tsamp is thenSint ≥ Sobs ·w
mt Tsys β; Sobs = pwi
G Np w∆f(5.1)(c.f. Lorimer & Kramer 2004), where Tsys ≃ 23 K is the system temperature of the multibeam system, G is the telescope gain (ranging 0.735–0.581 K Jy−1 from the central to outerbeams), and Np = 2 is the number of summed polarisations in the data. β is a factor oforder ∼1 that is included to represent signal losses due to system imperfections. Fig. 5.3,

we indicate our sensitivity to single pulses of various durations as a function of dispersion
measure, and in comparison to the sensitivities of PKSMB and BSB.
The 2-bit digitisation levels of the BPSR instrument and large number of frequency
channels result in the increased dynamic range capabilities over the previous Parkes analogue filterbank. In HTRU Survey data, each frequency channel is sampled at levels
defined l = 0, 1, 2, 3. This produces dedispersed time series with integer values ranging
0 to 2610. When processed using the dedisperse all software, for efficient data size
when written to disk, and to a lesser extent for efficient processing (see HTRU Paper 1),
input data values get transformed as (lNchan /16) − 128. In the absence of interference, themean bit level should be 1.5. The mean of the time series produced by dedisperse all
should therefore be µ ≃ −46.5, with a data maximum value of 35 (when all channels havel = 3). We made empirical measurements of the mean and standard deviation (σ) of data
judged by eye to not be strongly affected by interference. We found µ = −48 and σ = 2

for interference-clean files, in reasonable agreement with the expected mean. With thesevalues, we calculate the highest achievable SNR in the HTRU Survey data for pulses withw = tsamp to be (35 − µ)/σ = 41.5. This is in good agreement with the SNR of saturated
interference pulses observed in DM = 0 pc cm−3 time series. For the 1-bit, 96-channel

analogue filterbank, the maximum SNR may be calculated analytically. In this system,
data uninterrupted by interference has an equal probability of having bit level l = 0 (“off”)
or l = 1 (“on”); that is, Pon = Poff = 0.5. The data values in this system are describedby a binomial probability distribution, with the probability of being in a state with non
on-channels and noff off-channels (non + noff = Nchan = 96) given by:
Pstate =Nchan !
(Pon )non (1 − Pon )Nchan −nonnon !(Nchans − non )!

(5.2)

This function is a Gaussian with µ = 48 and σ = 5. The theoretical maximum SNR ofthis system at w = tsamp is then (96 − µ)/σ = 9.6. Again, this agrees with saturated

interference signals observed in data from this system. The differences in dynamic range

104

Chapter 5. Transients in the High Time Resolution Universe Survey

for these systems do not strictly impact the flux sensitivity of a single-pulse search, however
the HTRU Survey’s increase in dynamic range will affect the manual inspection stage byallowing detections of potentially very bright, narrow pulses to be more clearly discernible
from noise or non-Gaussian statistics in the observation. This effect is more acute forpulses of width close to the sampling time of each analogue filterbank survey.
The improved resolution of the HTRU Survey is clearly beneficial for the detection of
RRATs, which are known to have narrow pulse widths (e. g. McLaughlin et al. 2006), andother pulsars or extragalactic impulse emitters with durations w . 10 ms and dispersion
measures of DM . 360 pc cm−3 . For DM > 360 pc cm−3 , we do not expect to see a large
number of new RRATs for several reasons; first, we do not have a considerable increase
in sensitivity over the previous surveys for these DMs, and the number RRATs above thisDM represents only ∼10% of the known RRAT population. Additionally, for |b| < 5, the35-minute pointing duration of PKSMB searches afford a greater probability of detectionof low-pulsation rate events over our 8.5-minute observations of these areas, and we offer
minimal improvements for the capability of detection of these objects. At higher latitudes,
for the bulk of pointing positions in the HTRU Survey the number density of electrons in
the Galaxy is insufficient to produce dispersion measures much greater than 360 pc cm−3
(Cordes & Lazio 2002). For these reasons, we expect most new rotating radio transientand deep nulling pulsar discoveries to be at dispersion measures DM < 360 pc cm−3 . As
detailed in §5.4.1, this seems to hold true for the initial discoveries.

5.2.3

Current Processing Status

The current status of the HTRU Survey processing is summarised in the upper panel ofTable 5.1. The processing reported in this paper includes only pointings from the HTRU
intermediate and high latitude surveys. The survey coverage to date is not contiguous
over all galactic regions. For later interpretation of the galactic distribution of our discoveries, we indicate the total observation time of the processed pointings as a functionof sky position in Figure 5.4. In total, 22,273 and 1,718 beams have been processed outof the 95,056 and 443,287 total beams of the intermediate and high latitude surveys, respectively. Approximately 160 incomplete survey pointings of shorter duration have also
been processed. Nevertheless, the bulk of observing time reported in this paper has beenspent at galactic latitudes |b| < 15.

5.2. The HTRU Survey’s Single-pulse (SP) Search

105

Figure 5.4 A galactic map showing new HTRU Survey single pulse discoveries, and observing time spent per l, b cell. The stars indicate detections from which multiple pulses havebeen detected, and triangles indicate sources which have only yet exhibited one strong
pulse.

106

5.2.4

Chapter 5. Transients in the High Time Resolution Universe Survey

Candidate Ranking and Follow-up Strategy

Because telescope time is a valuable resource, the infrequent emission from RRATs andextreme nullers make it scientifically inefficient to perform extensive follow-up for all of
the candidate pulses discovered in our search, many of which will not be genuine celestial
objects. We employ a follow-up prioritisation scheme to provide efficient scientific returnwith available observing time. Based on the strength of the candidate in various panels of
the manual inspection plots and the number of detections at a similar DM in a pointing(recall Fig. 4.1 and Fig. 5.2), candidates are ranked multi-pulse (two or more strong pulses
detected at similar DM), strong (≥2 weak pulses, or one convincing pulse detected, thatis with a SNR > 7 and exhibiting a characteristic signature in the SNR vs. DM plot,
e. g. as explored by Cordes & McLaughlin 2003), or weak (candidate’s time series showsa detection at non-zero DM but is poorly supported by data in other inspection panels
and/or has SNR < 7). Weak candidates outnumber those ranked strong and multipulse by a factor of more than two. In scheduling follow-up time, all candidates areobserved for a duration equal to that of the discovery pointing. Strong and multi-pulse
candidates are of higher priority. If weak detections are not seen in the initial follow-up
pointing, they are not tracked in further follow-up observations. We do not include thesedetections in this report due to their highly uncertain nature, however we preserve them ina database for posterity, taking into consideration that further weak detections in future
pulsar surveys of the same sky area could cross-correlate their findings with our weak,unconfirmed detections (e. g. as will be possible with the HTRU Survey’s low-latitude
survey, and potentially with future Australian Square Kilometre Array Pathfinder/Square
Kilometre Array surveys). Multi-pulse and strong candidates not seen in initial follow-up
are observed again for a duration of three times the discovery pointing. We note that theinactivity states in nulling pulsars has a broad range of timescales; on one extreme, RRATs
have been observed to emit single pulses separated by up to 3.25 hours (McLaughlin et al.
2006), and on another extreme, emission from some pulsars has been observed to turnoff for up to 5 days. Our ranking levels are set such that these categories are highly
unlikely to contain detections of spurious noise. We include all strong and multi-pulse
candidates in this report whether or not they are detected in follow-up pointings, withthe strong caveat that these objects essentially remain unconfirmed, and care must be
taken in including these objects in statistical or categorical transients studies. Objects
not detected in follow-up observations are distinguished in Table 5.2 by a limit instead ofa measurement of χ̇; this reflects that non-detections of these discoveries are used only toplace limits on χ̇ for the object. Only one multi-pulse object has yet not been detected in5.3. Early Discoveries & Detections
PSRJ
RAJ
DECJ
P (s)
Sources with multiple-pulse detections
J0410–3107 04:10:39 –31:07:29 1.8785(2)J0837–2447 08:37:44 –24:47:48–
J0912–3848 09:12:27 –38:48:34 1.5262(1)J1014–4849 10:14:18 –48:49:42 1.5088(2)J1216–5027 12:16:20 –50:27:01
6.355(9)J1307–6703 13:07:41 –67:03:27 3.65120(8)J1549–5721 15:49:05 –57:21:37 0.7375(3)J1855–1558 18:55:24 –15:58:57 3.4532(1)J1925–1601 19:25:06 –16:01:00 3.8858(2)Sources with only one pulse detected
J1135–4925 11:35:56 –49:25:31–
J1424–5640 14:24:23 –56:40:47–
J1541–4218 15:41:55 –42:18:50–
J1709–4354 17:09:47 –43:54:43
–

107
χ̇

DM

weff

Smax

107
5
32
16
13
11
73
25
≤6.5

9.2(3)
142.8(5)73.3(5)87(7)110(20)47(15)
17.7(3.5)160(25)88(20)

18
1
6
21
9
32
4
65
10

470
420
190
140
130
70
210
50
160

≤1.3
≤7
≤7
≤7

114(20)27(5)60(10)
228(20)

9
22
4
3

120
110
150
240

Table 5.2 Properties of the new sources detected by this search. Columns: (1) Name basedon J2000 coordinates (Note, candidates from which only one clear pulse detection has yet
been made are listed separately. Such objects must be interpreted with care; see noteson these and the objects with limits only on χ̇ in §5.2.4); (2,3) J2000 right ascension anddeclination; (4) the best-fit period, where measurable, with the error on the last digit inparentheses; (5) pulsation rate χ̇ = Np h−1 ; (6) best-fit dispersion measure and error inpc cm−3 ; (7) observed pulse width at half-maximum of the brightest pulse in milliseconds;(8) peak flux density in units of mJy of the brightest detected pulse (calculated using Sobs
in Eq. 5.1).
follow-up observations. However, the strong candidates have a higher non-detection rate.
The yet-unconfirmed “strong” objects have names listed separately (in the lower panel) in
Table 5.2. In Section 5.4.2, we briefly addresses whether our discoveries are statisticallyconsistent with the expectation that we are more sensitive to objects with “off-timescales”
less than our observation length.
For objects of pulsation rate χ̇ ≥ 40 h−1 , we plan to maintain dedicated observations

to achieve phase-coherent timing solutions for these objects.

5.3 Early Discoveries & Detections
5.3.1

New Discoveries

Table 5.2 lists basic parameters of the 13 new objects discovered thus far using the HTRU
Survey single pulse pipeline. There are several objects worthy of specific note.
PSR J0410–3107, at galactic latitude b = −56◦ , this is the only confirmed single-pulse

108

Chapter 5. Transients in the High Time Resolution Universe Survey

discovery arising thus far from the high latitude survey. Estimating distance to the new
discoveries using their dispersion measures and the Cordes & Lazio (2002) electron density
model for the galaxy (hereafter NE2001), we find that this is also the most nearby object
discovered in our search (d = 0.51 kpc). Accordingly the DM of this pulsar is exceedingly
low, with DM = 9.2 pc cm−3 , lower than 98.9% of known radio pulsars.
PSR J0912—3848 was also detected in the HTRU Survey Fourier-domain search.
Bright single pulses were only detectable above the noise for .30 seconds in both thediscovery and follow-up observations. When the data are folded over pulses not detectedby the single event search, the pulsar remains detectable with a signal-to-noise ratio of >5;
this behaviour is not consistent with scintillation based on the scintillation timescale andbandwidth in the NE2001 model. The abrupt change in flux density and the occurrenceof these short-duration flares in both observations lead us to suggest they are intrinsic tothe star.
The pulses of PSR J1014–4849 were only detected in one cluster spread over 16 rotations of the neutron star. Like PSR J1825–33 of the previous chapter, no outbursts have
yet been detected in follow-up observations.
Broad-bandwidth amplitude modulation features are seen in the frequency-dependent
structure of the single pulses of PSR J1549–5721 (see Fig. 5.5), the bandwidth of which areinconsistent with the predicted scintillation bandwidth of NE2001; the observed bandwidthis larger by approximately four orders of magnitude. The origin of this feature is unknown.
However, despite the significant disagreement with the NE2001 prediction, it is likely that
if the detected emission is produced by a neutron star, the observed modulation is the resultof an unmodelled local clump or void of material in the interstellar medium that is inducing
scintillation to the observed degree. However, it would be negligent not to note that such asignal could originate from extraterrestrial intelligence; it is dispersed, periodic, relatively
narrow-band signals like these that are often targetted in ET intelligence searches. Future
confirmations of the spectral characteristics, and particularly timing of this object caneasily conclude its origins as neutron star or otherwise.
The predicted scatter broadening of PSR J1709–4354 (130 ms at 1.375 GHz for theNE2001 model) is much greater than the half-maximum pulse width detected here. The
singe pulse from this object had a signal to noise ratio of 10, a visible dispersion curve,a signal versus DM curve well-fit to the model of Cordes & McLaughlin (2003) for agenuinely dispersed pulse, and exists in a relatively interference-free observation. These
conflicting properties indicate that either the impulse’s origin is not celestial (e. g. of thetype presented in Chapter 6), or that the NE2001 scattering model for this galactic position

5.3. Early Discoveries & Detections

(a)

109

(b)

Figure 5.5 PSR J1549–5721: (a) A pulse stack of 16 single pulses. (b) The data afterfolding over the 16 stellar rotations shown in (a): a waterfall image (lower panel) withdedispersed and band-integrated pulse profile (upper panel).

Figure 5.6 A pulse stack showing single pulses of the periodically nulling pulsar J1855–
1558.

110

Chapter 5. Transients in the High Time Resolution Universe Survey

is incorrect. A redetection of this candidate would clarify the nature of this candidate,however follow-up has not yet been performed.
PSR J1855–1558 appears to be exhibiting periodic nulling, drifting, and mode-changing
behaviours (Fig. 5.6). Despite a high nulling fraction, this object’s relatively frequentemission renders it detectable in a Fourier search with a signal to noise ratio of ∼17 in the
8.5-minute survey and follow-up pointings.

5.3.2

Redetections of Known Pulsars and RRATs

The single pulse pipeline has detected approximately 55% of the known pulsars that could
be detected through Fourier searches of the HTRU Survey data. This offers a slight improvement in single-pulse detection rate over the search presented in the previous chapter,
perhaps a testament to the increased sensitivity to faint and narrow single pulses that thissurvey’s hardware affords. The single-pulse properties of these objects will be analysed infuture work, so we will not detail them here.
We have processed 24 observations with pointing positions within 6 arcminutes (ourapproximate half-power beam width) of published RRAT positions (i. e. from McLaughlin
et al. 2006, Keane et al. 2010, McLaughlin et al. 2009, Burke-Spolaor & Bailes 2010). Of
these 24, 13 RRATs were redetected. For 22 of 24 observations, the number of pulses seenfrom each object were consistent with the expected number based on previously publishedpulsation rates in the discovery observations of these objects (again, as drawn from the
BSB and PKSMB publications) and the observation length in our data. The two notably
different detection rates were of RRATs J1753–38 (BSB; 4 expected and > 15 seen) andJ1819–1458 (McLaughlin et al. 2006; 3 expected and 10 seen). PSR J1819–1458 has been
recently noted to exhibit small variations in pulsation rate (and larger ones associatedwith glitches Lyne et al. 2009), and the detection rate observed here is consistent withthe range in χ̇ reported by (Lyne et al. 2009). For PSR J1753–38, the higher detection
rate, detectability in a Fourier search with a signal-to-noise ratio of ∼13 (whereas it wasundetectable in a Fourier search of its original discovery data), and the slight increase

in Smax exhibited in our observations indicate that: 1) this pulsar’s emission appears tosimply be highly modulated, and does not have genuine deep nulls, and 2) our observing
position of the source is improved from that reported by BSB. Our positional offset was
0.1 degrees, suggesting an improved position of right ascension and declination 17:53:09,–38:52:13 (J2000).

5.4. Discussion

111

5.4 Discussion
5.4.1

Our Discoveries in Archival Pulsar Surveys

As noted in §5.2.2, for the regions overlapping with previous surveys, we expected to

discover new objects mainly at DM . 360 pc cm−3 . This appears to have been met,
however several of our detections appear to be of sufficient brightness that one would haveexpected a detection in archival data. All of our discoveries except for PSR J0410–3107
and PSR J0837–2447 lie in regions previously covered by the surveys of Jacoby et al.

(2009), Edwards et al. (2001) and Manchester et al. (2001). The presence of pulses inarchival data—note that these surveys were performed 8-12 years ago—can add valuable
information about pulse rate changes over time, and potentially add data points to theirtiming if a phase-coherent timing solution is obtained. We inspected pointings in thesesurveys within one half-power beam width of our objects’ positions to determine thedetectability of our discoveries in the archival data. PSR J1307–6703 is the only one ofour discoveries which exhibits clear single pulse detections when the archival data arededispersed at the appropriate DM. We detect seven pulses of SNR > 5 in the 35-minutearchival observation for this object (giving a χ̇ consistent within a factor of two of thatderived from the solitary detection in the HTRU Survey discovery pointing). It is from thearchival data that the period quoted in Table 5.2 was determined. While PSR J1855–1558
is not detectable in a Fourier search, it is marginally detectable when the archival data
are folded over the period given in Table 5.2.
We can investigate the reason for the non-detection of the remaining objects by firstestimating the signal loss due to the use of the analogue multibeam filterbank system usedby BSB and PKSMB, calculating the SNR that a pulse of equivalent width, DM, and ·Speak(as listed in Table 5.2) would have shown in the archival data. This analysis suggests thateven the brightest detected pulses in our data would result in only a marginal (SNR . 6)
detection in the analogue filterbank data for PSRs J1135–4925, J1541–4218, and J1709–
4354. For two out of four of the remaining objects at |b| > 5◦ , for which the observation

length of the archival data was 4.4 minutes, the non-detections could be accounted for by
the low probability that a pulse would occur during the time span of the observation for
objects with χ̇ . 13.5 h−1 . The remaining |b| > 5◦ objects, PSR J0912–3848 and PSR
J1014–4849, had clusters of sequential pulses rather than a smooth distribution of single

pulses. Although the per-hour pulsation rate is high for these objects, the average durationof on-activity is short, the spacing between sequential pulse outbursts is &8 minutes. This
indicates that the non-detection of pulses in the archival data is consistent with a decreased

112

Chapter 5. Transients in the High Time Resolution Universe Survey

probability of the occurrence of a pulse outburst during the archival observations (if there
were they should have resulted in detections of SNR ∼8 and 11 for PSRs J0912–3848 andJ1014–4849, respectively). The effect of on- and off-timescales for object detectability will
be discussed further in Sec. 5.4.2.
The only remaining undetected objects are PSR J1549–5721 and PSR J1424–5640. We
have not yet performed follow-up observations on these objects, and so the archival data,in which we would have expected to see detections of SNR > 7, serves only to place limits
on the pulsation rate and duration of off-activity in these objects. For PSR J1549–5721,
the non-detection in two archival pointings places a strong limit (zero pulses in ≥70 m) on
these values.

5.4.2

Activity Timescales in Sparsely-emitting Neutron Stars

As reviewed in §4.5.1, the nature of RRATs, in particular their relationship to averageradio pulsars, is still somewhat a matter of debate. As explored briefly by SBS and
Keane et al. (2010), and in detail by Miller et al. (submitted), most objects discovered insingle pulse searches do appear to be genuinely “turned off” when they are not detected,
rather than being distant pulsars with atypically long-tailed pulse energy distributions;
one can discern these classifications by single-pulse energy statistics and the identificationof whether RRATs’ detectable pulses show a distinct log-normal distribution (Miller et al.
submitted).
For the objects exhibiting genuine nulls, the possibility of a clearly distinguished“RRAT” class of neutron stars is beginning to break down, based on mounting evidence
that RRATs simply represent an extended tail of deeply-nulling objects in the nulling
population of pulsars. This is supported indirectly by the lack of distinction of RRAT andpulsar Galactic and pulse width distributions (SBS) and further by the pulse energy distributions of RRATs, which as noted above tend to obey similar log-normal distributions
to average (nulling and non-nulling) pulsars. Furthermore, Keane (2010) note that the“classic” definition of the RRAT class implied by the original McLaughlin et al. (2006)
publication is a neutron star for which r = mSP /mFFT > 1, where mSP is the SNR of thebrightest single pulse detected in an observation, and mFFT is the SNR of a Fast Fourier
Transform analysis for the pulsar in the same observation. This leads directly to theinferrence that in fact whether a source is defined “RRAT” or “pulsar” is arbitrary, andhighly dependent on survey pointing length and pulsar period (McLaughlin & Cordes 2003,
Keane 2010), further obscuring RRAT intermittence as an exclusive physical phenomenon.
For selected objects, However, we do note that observed post-glitch behaviour of RRAT

5.4. Discussion

113

J1819–1458 (which showed a net decrease in spin-down luminosity post-glitch, rather than
the increase usually observed in pulsars Lyne et al. 2009) does set this particular object
apart from the pulsar population and drive the “exhausted magnetar” hypothesis put
forward by Lyne et al.
Nevertheless, given these facts we seek to more accurately incorporate our discoveries
and RRAT populations in general into the range of nulling and emissivity timescales
exhibited by radio pulsars. We explore a relevant “intermittence parameter space” here,
and review in Sec. 5.4.3 what effect survey parameters have on the selection of various
populations in this space.
As with the objects discovered in the search in Chap. 4, approximately 1/3 of oursingle pulse pipeline discoveries exhibit distinct periods of on-activity, marked by brightsequential pulses that are separated by longer intervals of either genuine nulls, or a decrease in intensity level sufficiently large that the object is undetectable through single
pulse or periodicity search techniques. The pulsation rate, χ̇, that is typically quoted
for single pulse discoveries poorly represents the emissivity of such objects and provides
insufficient information to assess the probability of detecting them during reobservation.
We return now to the “windowing” description of pulsar and RRAT emissivity that was
introduced in the previous chapter (recall Fig. 4.7), noting that the on and off states ofnulling pulsars often show characteristic timescales (Herfindal & Rankin 2007, Redman &Rankin 2009, Wang et al. 2007) given by ton and toff , respectively. Quantifying intermittence timescales in this way is more physically representative for pulsars of various nulling
fractions, better reflects the timescales associated with possible windowing phenomena,
and allows a more accurate exploration of selection effects for surveys of various length(see §5.4.3). Here we use a definition of toff that is similar to the “null length” of Wanget al. (2007). We define toff as the average time between the first pulse whose signal drops
below a set threshold (we use SNR > 5) and the next above that threshold. We define
ton as the average number of pulses (Non ) above the same threshold, times the rotational
period of the object. Obviously, such an analysis is only valid for genuine nulling pulsars, and uninterrupted data spans of T ≫ ton + toff containing single pulse detectionsof high significance would provide more accurate measurements of these quantities; forour discoveries, most observations provide robust single pulse detections, although we areinsensitive to timescales ≫ 9 minutes. We nevertheless make estimates of (or place limits

on, in observations where no timescales are measureable because they appear greater thanour data span) the average ton , toff for our discoveries based on all available data.
The nulling fraction of a pulsar is only valid in this analysis when measured over a

114

Chapter 5. Transients in the High Time Resolution Universe Survey

timescale far exceeding one activity cycle of the object, which is given by a length ton +toff .
Below we generically represent the nulling fraction measured over “infinite time” as f∞ ,where accordingly, the pulsar’s on fraction measured over a time T ≫ ton + toff is givenby ξ = 1 − f∞ . As an illustrative example, the pulsar PSR B1931+24 exhibits periods ofactivity lasting ∼5-7 days, separated by nulls of ∼30 days (Kramer et al. 2006). While thenulling fraction measured over short observational timescales is therefore typically either
0% or 100%, we find that f∞ ≃ 85%.
In Figure 5.7, we plot ξ as a function of toff for the single-pulse discoveries from
HTRU, PKSMB, BSB, and Deneva et al. (2009). We also plot all nulling pulsars analysedby Wang et al. (2007) that have a non-zero nulling fraction. The Wang et al. sample
was selected from the periodicity-discovered pulsars of the same survey as the PKSMB
searches, and has poor sensitivity to nulls of ≪ 30 s due to the averaging performed over

10-30 s timescales. The dashed grey lines in this figure show contours of constant ton . Fora pulsar of period P , it is not straight-forward to identify on- or off-activity timescalesof duration t < P ; the shaded region below ton = 100 ms indicates the area for which ton
is shorter than the period range of single-pulse search discoveries that emit at Non = 1(the shortest such period, to the author’s knowledge, is the P ≃ 400 ms pulsar J1753–12
presented in Chap. 4).

The critical point illustrated by this plot is the continuous transition between the
Wang et al. (2007) nulling pulsars and the population discovered by single pulse searches.
McLaughlin & Cordes (2003) derive that neutron stars of period P will be more readily
discovered in a single-pulse search (i. e. having SNR ratio r = mSP /mFFT > 1, as with thep
RRAT as defined by Keane 2010), when ξ < 2 P/T . For the HTRU intermediate latitude

survey pointings, pulsars with periods P < 6 s should therefore be discovered more readilyin the Fourier pipeline at ξ > 0.2, and thus it is not surprising that no HTRU intermediate
latitude survey points lie above this line in Fig. 5.7. For the T = 35 min pointings of
the Parkes Multibeam survey (Manchester et al. 2001), r > 1 when ξ < 0.02 to 0.11,
considering a period range 200 ms to 6 s. Appropriately, this appears to be the region in

Fig. 5.7 where the nulling pulsars give way to the single-pulse discoveries. It is clear herethat the on and off timescales, and the nulling fractions of the radio pulsars discovered insingle pulse searches exhibit a smooth distribution over many orders of magnitude with no
obvious distinction of the “RRAT” population represented by the single pulse discoveries.
We do not have sufficient information to evaluate here whether the probability density
function of nulling fractions for single-pulse detections are consistent with an extrapolationof Fig. 7 of Wang et al. (2007). However, a rigorous nulling fraction analysis for bright5.4. Discussion

115

1
f∞=50%

0.1
f∞=95%BSB
PKSMB
Deneva et al. (2009)
Reported here
Nulling pulsars
B1931+24

t on

ξ

0.01
=
0
10
s

m

f∞=99.9%

0

10

s

s

10000
toff (seconds)

10

HTRU-lo

=

=

PKSMB

t on

t on

1000

s

100

1

10

=

0.0001t on

HTRU-med

0.001

100000

1e+06

1e+07

Figure 5.7 Here we show the on fraction, ξ, against the mean null length for single-pulse
search discoveries and the pulsars of Wang et al. (2007). All elements highlighted inmaroon indicate relevance to the HTRU intermediate latitude survey; the squares show
single pulse pipeline discoveries, while maroon circles indicate previously-known objects
which were redetected in the HTRU intermediate latitude survey data (§5.3.2). Points
with arrows designate an object for which ton has been measured but only a lower limit on
the null timescale is known, and points with flat-headed arrows represent objects for which
only one pulse has been detected (we have assumed P = 8.5 s for these objects, therefore
the plotted point is an upper limit to ξ and a lower limit to toff ). The solid green lines
indicate lines of constant nulling fraction as measured over infinite time (f∞ , see §5.4.2),
and the dashed gray lines show constant contours of ton . The vertical dotted lines show
the single-pointing observation length for the survey of PKSMB, and for the HTRU high(hi), intermediate (med), and low (lo) latitude surveys (see §5.4.3). The shaded region
below ton = 100 ms indicates the area for which ton is shorter than the period range ofsingle-pulse search discoveries, and is therefore ill-defined. B1931+24 is the “intermittent
pulsar” published by (Kramer et al. 2006), and discussed in Sec. 5.4.3.

116

Chapter 5. Transients in the High Time Resolution Universe Survey

pulsars in the HTRU intermediate latitude survey could address this, and is planned forthe near future.

5.4.3

Detectability of Intermittent Neutron Star Populations

The distribution of objects in Figure 5.7 is influenced by a number of selection effects,
which we explore here both to investigate the underlying distribution of activity cycles
in neutron stars, and to determine the population to which the three HTRU Survey
components will be sensitive.
Considering the general detectability of pulsars in this phase space, the probability
that at least one pulse will be emitted by a bright pulsar during an observation of length
T is given by:
Pem =



0




1




(T − ξT + ξtoff )/toff

for f∞ = 1
for T > toff

(5.3)

for T ≤ toff(note, however, that this probability is generally representative but will not always be
strictly true, as the distribution of ton and toff for a single object is not a delta function).
The vertical lines in Fig. 5.7 therefore mark the null length below which a sufficiently brightemitter has a probability of unity that it will be detected in the Parkes Multibeam survey, the HTRU intermediate latitude survey, and the (recently commenced) low-latitude
survey, respectively. At null lengths greater than the observing time for these surveys,
the probability of detection quickly decreases. Accordingly, given a population of pulsars
with a flat underlying distribution in log(toff ), we would expect to have a higher discovery/detection rate of objects with toff < T . Focussing on the objects detected by the
HTRU intermediate latitude survey (highlighted in maroon in Fig. 5.7), this appears to
be true, with a ratio of 7:13 for objects with toff ≥ 9 min to objects with toff < 9 min.
We point out that the object of the highest toff discovered in each of the PKSMB, BSB,

and HTRU intermediate latitude surveys is roughly a factor of three greater than thecorresponding survey’s length. This corresponds to Pem & 0.33 at all nulling fractions.
Equation 5.3 indicates that for toff < T , the probability a source would be detected isone, and for toff > T , it is directly proportional to ξ. As such, if the underlying neutron star
population has a flat distribution in both ton and toff , there appears to be a deficit of Parkes
Multibeam detections in the region 100 s < toff < 35 min, ton > 100 s. For objects with
ξ & 0.1 (that is, those theoretically detectable in a Fourier-search), there is no explicitly
stated reason that Wang et al. (2007) should have selected against this range of timescales.

5.4. Discussion

117

The paucity of objects occupying this region could be the combined result of a number ofpossible effects. Nulling behaviour may have not yet been either observed or recognised,however we would expect this to only be the case for objects with ton & 35 min, which
encompasses only a small fraction of the region in question. It is possible that some low orintermediate nulling-fraction objects with toff ≃ 35 were outright rejected from the survey
due to a non-appearance in follow-up observations. We consider this an unlikely cause,
however, as the 6-minute grid pointings and subsequent 35-minute follow-up pointings(in case of a non-detection in the grid observation, Manchester et al. 2001) would haveprovided a sufficiently high redetection probability, particularly for objects with toff ≪ T .
We note that for objects with toff close to 35 minutes, the survey’s follow-up observations

might contain only a few pulses from the objects, which are detectable in single pulses,
rather than in the Fourier search used to assess the follow-up pointings in the survey.
However, this effect still does not account for the notable deficit in the region in question.
It is furthermore possible that the lack of objects in this region reflects an actual dropin the neutron star population for objects with ton & 300 s. This supposition is questioned,
however, supported by the existence of the “intermittent pulsar” B1931+24 reported by
Kramer et al. (2006). This object is distinctly isolated in this space, although unpublisheddetections of further objects occupying a similar region of Fig. 5.7 have been noted (Keith2007). The existence of these objects suggests that either there is a bridging distributionof neutron stars which we have not yet identified due to an unrecognised effect of selection,or that in fact the intermittence of these objects is produced by a physically distinct mechanism. Immense observing timescales are required for a survey to have a high probabilityof detecting a population between the ton . 300 s objects and PSR B1931+24-like pulsars.
Because data storage and analysis becomes a problematic for pulsar surveys of longer
timescale, we suggest that the brightest pulsars in this population may be more readily
uncovered by the “transient-imaging” surveys that are being performed and planned onarrays; these surveys can sample timescales down to the correlator averaging time (typically &10 s) and up to several hours to days. However, for pulsars with ton less than thesetimescales, pulsar surveys remain the most robust probes of intermittent populations.

5.4.4

Discovery Forecast for the Full HTRU Survey

The rate of new HTRU Survey single pulse discoveries, particularly those in overlapping
regions of previous surveys, hint at the potential for the full HTRU Survey to uncover many
new examples of sparsely-emitting neutron stars. For the intermediate latitude survey, we
have detected a total of 26 new and known single-pulse emitters, suggesting detection

118

Chapter 5. Transients in the High Time Resolution Universe Survey

rates on the order of 3 × 10−3 deg−2 at these latitudes. We have processed roughly 23.5%of the intermediate latitude survey to date, and if our discovery rate continues, the full
HTRU intermediate latitude survey data should contribute an additional estimated ∼50
new discoveries of transient neutron stars. We note that the sky distribution of neutronstars should have a higher density at lower galactic latitudes, and as seen in Figure 5.4
we have not yet processed most of the pointings at latitudes closest to b = 0. At higher
latitudes we have only processed a small fraction of the survey, and while tempting totake an extrapolated value as an upper limit (implying the HTRU high latitude survey
may return up to 250 new transient emitters), we note that our single discovery in only
0.39% of the high latitude survey implies a sky density of ∼2 × 10−3 deg−2 , higher than
expected when compared to the detection rate at lower latitudes. Therefore, the detection

rate is expected to be much less. Regardless of the discovery rate, however, the highlatitude survey will play an important role in characterising the galactic distribution ofdeep nulling pulsars.
While we have not yet processed any data to determine the discovery rate of the lowlatitude survey, the explorations of Sections 5.4.2 and 5.4.3 accent the timescales of neutronstar intermittence that this survey, with its 70-minute pointing length, will explore. Thissurvey will mark a significant increase in detection rate for objects with 35 < toff < 70 min,and may be realistically expected to discover objects with null lengths up to toff = 3.5 h,
if such objects exist (as deduced from the point in §5.4.3 which noted that transientssurveys have all been able to detect objects with toff values of up to 3 times the survey
pointing length). This survey will also provide data that may be used to investigate thedeficit of low-nulling-fraction objects with ton > 300 s. Care must be taken during boththe single-pulse and periodicity searches, analysis, and follow-up, such that if these objects
do exist, they are not selected against. This will involve, for one, careful analysis (withboth Fourier and single-pulse searches) for follow-up pointings.

5.5 Summary and Conclusions
We have presented the methods of and initial discoveries for the single-pulse analysis of
the High Time Resolution Universe Survey. We outlined the design of the HTRU Survey’s single-pulse pipeline, which functions efficiently in concert with the Fourier-analysis
pipeline; the single-pulse analysis furthermore employs a “friend of friends” single-event
recognition algorithms and performs automated interference rejection based on the dispersive and multi-beam signature of single events. The new digital backend used to collect
HTRU Survey data has afforded a factor of up to ∼5 times improvement in sensitivi-

5.5. Summary and Conclusions

119

tiy over previous surveys in overlapping galactic regions, and offers the most significant
improvements for short (sub-ms duration) pulses at low (DM < 360 pc cm−3 ) dispersion
measures.
Analysis of 23.5% and 0.39% of HTRU intermediate and high galactic latitude survey
data, respectively, has resulted in the discovery of 12 and 1 new neutron stars, respectively.
Much of the survey pointings covered galactic regions that were previously surveyed andsearched for single pulses (McLaughlin et al. 2006, Keane et al. 2010, Burke-Spolaor &Bailes 2010); 11 of our new discoveries lie within these regions, and for the nine of thesethat were not visible in the archival data, their non-detection was consistent with either
the signal degradation due to the use of the previous wider-channel analogue backend, orwith the improbability of a pulse being emitted in the archival survey due to a long nullcycle.
Finally, we investigated the distribution of nulling and emissivity timescales for the
new HTRU Survey single-pulse neutron star discoveries and redetections, and for the
RRAT/nulling population in general. We found that low/intermediate nulling-fraction pulsars and single-pulse search discoveries exhibit a continuous distribution across null/activity
timescales and nulling fractions, building on evidence that RRATs represent a tail ofextreme-nulling pulsars. We found that there is a deficit in objects with emissivity cycles
longer than ∼300 seconds at intermediate and low nulling fractions which is not readily

explained by selection effects, and note that the HTRU deep low-latitude survey will becapable of exploring whether this deficit is natural or an effect of selection. Lastly, we estimated that the full HTRU Survey is capable of more than (at least) doubling the known
deep-nulling pulsar population, and will explore the neutron star population with nulling
fractions exceeding 99.99%, and null lengths lasting up to 3-4 hours.

6
Terrestrial Impulses that Mimic
Frequency-dependence of Cold Plasma Dispersion

I would rather learn from one bird how to singthan teach 10000 stars how not to dance.
—e. e. cummings
In this chapter, we report on the serendipitous discovery of “Perytons” in the archival
data search reported in Chapter 4. These surprising discoveries are transient bursts whichshow several confounding properties: their frequency sweep mimicks the cold plasma dispersion relation typically taken as a sure indication of a celestial origin, but other propertiesof the bursts provide solid evidence that their origin is terrestrial. The bursts furthermore
have best-fit dispersion measures which cluster around the DM of the Lorimer Burst(§1.4.4; Lorimer et al. 2007). This discovery has significant implications for transient
search methods and the suitability of single-dish detectors to make definitive detectionsof single impulses. We perform a detailed investigation into the properties and origin of
these signals, and discuss whether the Lorimer Burst and the Perytons have a genesis in
the same phenomenon. While it is not yet clear what is causing the bursts, we propose ascenario in which the swept-frequency emission could be produced by a lightning-related
plasma phenomenon in Earth’s atmosphere.

6.1 Introduction
As described in §1.4.4, three years ago the “Lorimer Burst” (LB hereafter) was thought tobe the first discovery of a rare, impulsive event of unknown extragalactic origin (Lorimer

et al. 2007). The extragalactic interpretation was based on the swept-frequency nature of
the event, which followed the dispersive delay expected from an extragalactic pulse.
121

122

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data
Since its discovery, some doubt has emerged over the extragalactic origin of the LB.

The high intensity of the LB (∼100 times the detection threshold) implies that searches ofsimilar sensitivity should detect fainter events, if such bursts are isotropically distributed
throughout the Universe and/or have an underlying intensity distribution that is typicalof astrophysical phenomena. The failure of searches to find similar events indicates thateither this is not true, that the astrophysical process that caused the Lorimer Burst is
extremely rare (< 2.8 × 10−5 hr−1 deg−2 for fluxes S > 300 mJy, based on the data from

LB, Deneva et al. 2009, Keane et al. 2010, and Chap. 4), or the burst’s properties were
misinterpreted.
In this chapter, we report on the detection of 16 pulses that exhibit a dispersion-like,frequency-swept signal with similar sweep rate to the Lorimer Burst. However, these new
events are clearly of terrestrial origin with properties unlike any known sources of broadband radio emission. We detail the properties of the new detections and scrutinize themin comparison to the LB. We explore atmospheric and man-made origins for such events,
and propose a model in which a lightning-related Langmuir mode-conversion process canreproduce the properties of the detected impulses and the LB. We then discuss methods
that can be used to distinguish an astrophysical or terrestrial origin for future detectionsof frequency-swept pulses, and provide both difinitive origins for these events, and a solidstatement on the extragalactic nature of future detections.

6.2 Data Set and SearchThis search and data sets used to find 15/16 of these discoveries was described in §4.2,
however our search here was augmented by slight differences in interference mitigation

filters; we inspected a candidate if it either a) was detected at a signal-to-noise ratio of > 6
in less than 9/13 of the receivers, b) had a signal-to-noise ratio of ≥ 20, or c) showed a DMof higher than 250 pc cm−3 . The final candidate was discovered in the search of two surveys(Crawford et al. 2006, Roberts et al. 2002; their observing parameters are detailed in theirSec. 2 and Table 1, respectively), and provided via private communication by F. Crawford.
The search methods used for that data are described in (Crawford et al. 2007, 2009); thesearches applied no interference excision based on multiple-beam detections. These weregenerally surveys of lower galactic latitude, therefore DMs up to 1000 (Crawford et al.
2006), 2000 (for target AX J1826.11300 of the Roberts et al. survey) and 2500 pc cm−3
(for the other targets of Roberts et al.) were searched. Candidates from these surveys
were inspected one beam at a time by eye to identify dispersed pulses.
In total, the data spanned over the years 1998 to 2003, all taken with the 20cm multi-

6.2. Data Set and Search

123(a) Peryton 08 in 13 beams(b) Peryton 08

Figure 6.1 Spectrograms and time series for Peryton 08. (a) Dedispersed time series
showing Peryton 08 in the 13-beam multibeam receiver as the beams are distributed on
the sky. (b) Dedispersed time series and spectrogram. The black lines trace the best-fit
dispersive delay for this detection. Data from the 13 beams have been summed to enhance
the signal. Frequency channels with known interference have been blanked.

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Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data
8
Lorimer burst

7
6

Number

5
4
3
2
1
0
0

100
200
300
400
500
Extrapolated delay across 288 MHz band at 1.4 GHz (ms)Figure 6.2 Distribution of fitted dispersive delays. The x-axis shows the total time to crossthe observing band, calculated from each pulse as a best-fit dispersive sweep. An arrow
indicates the delay of the Lorimer Burst. The dotted line indicates the delay below which
we had uneven search criteria (see §6.2).
beam receiver installed at Parkes Telescope.

6.3 New Discoveries and Their Properties
6.3.1

Flux and Spectral Properties

Our search revealed 16 pulses with two striking features that distinguish them from all others in the data: an apparent δt ∝ ν −2 delay of a magnitude implying an extragalactic origin

in the telescope’s pointing direction, and a simultaneous occurrence in all 13 telescope receivers at relative intensities of less than a factor of four (Fig. 6.1). When a dispersive
delay is fit to each detection, the values cluster about a net band delay ∆t = 360 ms,
indicating a close connection with the LB at ∆t = 355 ms (Fig. 6.2). The LB’s reported
sky position was below the horizon for several detections, therefore the pulses could nothave come from the same extragalactic source. Below, we give evidence that the 16 signals
have a terrestrial origin.
′

The 29 separation between each receiver’s beam position and > 20 dB attenuation′

beyond 30 from each beam center render it impossible for an on-axis, pointlike signal
to appear in more than 3 beams at similar intensity (Staveley-Smith et al. 1996, Hunt

6.3. New Discoveries and Their Properties

125& Wright 1992). We consider it unlikely that the source is on-axis and extended, basedon the lack of delay between the beams (requiring a distance ≪1000 km from the dishand a large emission source) and the matching spectral characteristics in all beams. It
is most likely therefore that our detections were made through a sidelobe of the Parkes
antenna, and based on the consistency of signals in the beams, in each case the emitter was
positioned ≫5 degrees from the telescope’s pointing direction. Consequently: 1) we havean minimum-senisitivity field of view of ∼20000 deg2 , a detection rate 2.3×10−7 deg−2 hr−1

(0.1 day−1 with our observing system), and poor accuracy for emitter localisation, 2)
the pulses are subject to frequency-dependent dropouts and scattering from multi-path
propagation—visible in Figures 6.1(b) and 6.5 and explored below—and 3) the source(s)’intrinsic flux density, had we pointed directly at it, is a factor of 2500-850000 greater thanthe detected value (0.8 < Sintrinsic < 272 kJy for the brightest detection, 0.1 < Sintrinsic <34 kJy for the faintest; see Table 6.1).

6.3.2

Evidence for a Terrestrial Origin

We conclude a terrestrial origin for these bursts based on their extreme brightness andtwo other features. First, some exhibit deviations from a model dispersive delay: e.g.
the sharp kink at 1465 MHz in detection 06 (Fig. 6.5), and subtler deviations in otherdetections (see the χ2 listing in Table 6.1). Despite a trend mimicking that expected fromdispersion, such deviations decisively distinguish the pulses’ frequency-dependence from adelay induced by interstellar propagation. Hereafter we distinguish these detections with
the name “Perytons,” representing the non-dispersive, highly swept, terrestrial signals
exhibited by the pulses1 (Fig. 6.3).
Second, the temporal distributions of the signals strongly imply a terrestrial origin(Table 6.1). All detections occurred during daylight, primarily mid-morning. Elevenappeared in one 4.4-minute observation followed by another 0.5 hours later (we regard
these as non-independent), while the remaining events occurred in isolation. Four of thefive independent detections appeared in a 3-week period in late June/early July spread
across the years 1998 to 2003, coinciding with the peak of Australian mid-winter. Theirtime distribution appears to follow a non-random both annual and daily cycle; we tested
the probability that 4/5 events would occur in June/July given an underlying random
distribution by running a Monte-Carlo simulation based on the monthly hours observed.
This test resulted in a confidence of P = 0.997231 of a non-random annual distribution.
1

The name is chosen from mythology to be unassociated with an exact physical phenomenon, due tothe ambiguous origin of the detections; Perytons are winged elk that cast the shadow of a man.

126

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data

Peryton
ID #
01
02
03
04
05
06
07
07a
08
09
10
11
12
13
14
15
LB

UT
Y-M-D-h:m:s
98-06-23-02:03:44.91
98-06-23-02:04:06.75
98-06-23-02:04:28.84
98-06-23-02:04:36.84
98-06-23-02:05:17.77
98-06-23-02:05:39.50
98-06-23-02:06:01.81
98-06-23-02:06:24.13
98-06-23-02:06:31.89
98-06-23-02:07:27.70
98-06-23-02:07:49.78
98-06-23-02:34:53.63
98-06-25-05:26:49.13
02-03-01-01:25:38.88
02-06-30-02:10:29.38
03-07-02-00:09:23.96
01-07-24-19:50:01.63

θz
(deg)
33.34133.28833.23533.21633.11833.06633.01332.96032.94132.80832.75529.73825.44534.51928.46544.09242.419θa(deg)
136.657
136.692
136.728
136.740
136.807136.843
136.879
136.916136.930137.023137.061
136.640
141.515320.875
189.173000.631183.315

∆t
(ms)
381.9
352.6
362.0
356.4
354.4
343.1
363.0
363.9
369.6
328.9
349.7
360.1
363.9
207.0
203.2
378.1
354.5

χ2
7.0
2.5
2.1
2.8
1.5
8.0
2.4
—
4.6
—
4.7a
1.9a
0.8
1.3
2.4
4.9
1.6

w
(ms)
35.2
46.9
31.2
35.2
35.2
31.2
39.1
32.1
39.1
43.0
31.2
46.9
39.1
31.3
39.1
39.1
15.6

Sdet
(mJy)
90
90
90
100
70
70
80
40
100
60
60
320
110
110
240
220
30000

Table 6.1 Columns: (1) Chronological ID; 07a was discovered after summing the 13-beam data,thus has a non-standard index. The Lorimer Burst is given for reference; (2) Arrival time at
1516.5 MHz; (3,4) Telescope zenith and azimuth angle, respectively, at the time of detection; (5)Extrapolated best-fit DM delay across the band; (6) Reduced-χ2 for a δt ∝ ν 2 fit, based on theevent’s time of arrival in 48 MHz sub-bands (if S/Nband > 5), timed against an analytic modelof the event’s dedispersed profile at the best-fit DM. Events 07a and 09 had no sub-bands of
S/Nband > 5; (7) Event width at half maximum after dedispersing at the best-fit quadratic delay;(8) Detected single-beam peak flux (Eq. 1.14). The Perytons’ intrinsic flux is much greater thanreported here (see main text).
aThis value would decrease without the presence of strong interference in the observation.

6.3. New Discoveries and Their Properties

127

Figure 6.3 An artist’s depiction of the beast that lends our bursts their name. Image
provided by Katherine N. Capach.

128

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data

A similar test based on the time of day distribution of observations and detections gave
a probability of P = 0.999046 that 4/5 events would occur in the UT range 0–3. These
cycles are strongly suggestive of either a climate/weather-related effect, or a man-madeorigin for the emission.

6.4 Discussion
6.4.1

Signal Origins

It is unprecedented for non-astrophysical emission to exhibit such drastic frequencydependent delays in the 1 GHz band. Given the daytime occurrence of the Perytons,
we first explored the possibility of the signals as man-made. The continuous emission
across the legally protected 1400-1427 MHz band suggest that the signal is not intentionally transmitted (although do not rule out the possibility); additionally, the combination ofa lack of regular periodicity, the broad pulse widths (30-50 ms), and broad-band emission
preclude a radar origin. Man-made emission that is unintentionally transmitted arises often from on-site electronic hardware failure. This does not appear to be the source of thisemission, however, based primarily on the amplitude modulation seen in all the detections.
These amplitude-modulated temporal or frequency structures show conformity across the
spectrum in all 13 beams for each burst. Assuming the modulation is attributable tomulti-path propagation effects (that these detections will necessarily show, as noted in§6.3 above), the incoming wavefront must not decorrelate over the physical size of thetelescope’s feed horns (1 meter), to allow the similarity of the modulation structures inall 13 beams. The diffractive scale (s0 ), therefore, must likewise be ≥ 1 m. Taking thecharacteristic bandwidth of the modulation structure in all pulses to be ∆f ∼ 10 MHz),and based on the center frequency f = 1374 MHz, we place a lower limit on the distanceto the object(s) and scatterer(s) of 2π(s0 f )2/c∆f > 4 km. This suggests that we have detected emission from the horizon—well off-site from the telescope grounds—and provides
the strongest argument against on-site hardware failure as the source. The quasi-annual
cycle and the spectral complexity with a 6-year persistence of the signal also argue against
local hardware failures as the emission’s origin.
While we cannot rule out an unknown satellite-based origin for the signals, it is pertinent to consider the possibility that the Perytons were caused by a natural terrestrial
source. Given the inconsistencies of the signals with man-made origins, we did explore
this possibility, considering processes that can produce non-dispersive, swept emission
with sweep rates of ∼1 GHz/s.

The emission requires a process of finite-bandwidth

6.4. Discussion

129(∆f < 25 MHz) signal to progressively change in its center frequency, for instance cyclotron emission in a time-varying magnetic field or the progressive incitement of plasma
oscillations in regions of differing plasma density.
The latter of these processes occurs in type III solar bursts (e. g. Loughhead et al.
1957b), in which we find a well-suited explanation for our bursts in a Langmuir wave modeconversion mechanism, in which a flare propagates through a plasma of changing density,
inducing electromagnetic waves at the plasma frequency (or its second harmonic) andresulting in a flare swept with attributes characterised by the velocity of the flare and theshape of the density gradient along the flare propagation path. Such emission, for instance,
arises from the type III solar bursts at sub-GHz frequencies (Loughhead et al. 1957b) notedin §1.4.3. Emission via this mechanism in our observing band requires electron number
densities of order 1016 m−3 . Such densities are transiently reached by ionised channels

that accompany electrical storm phenomena, ranging >1010 m−3 for upper-atmospheric”red sprites” (Dowden et al. 2001), up to 1023 m−3 for cloud-to-ground lightning (Rakov &Uman 2003). A super-ionisation event of this magnitude would be required for terrestrial,
swept plasma-oscillation emission in our band.

6.4.2

Investigation of Atmospheric Plasma Emission Origins

We investigated the suitability of an atmospheric Langmuir wave emission model to explain
our impulses by considering the simple case of propagation vertical, lightning-induced
plasma channel at 100% atmospheric ionisation.
An atmospheric plasma emitting at a radio frequency f (i. e. an electron plasma frequency ωp = 2πf ), must have a sufficiently high level of ionization to have a free electronpdensity, ne , obeying f = ne e2 /mǫ0 /2π. Emission swept through our observing band,from f1 = 1516.5 MHz to f2 = 1228.5 MHz, requires ne1 = 2.853 × 1016 /m3 for f1 , and
ne2 = 1.872× 1016 /m3 for f2 , or one fourth of these values if the detected emission is a sec-

ond emission harmonic (2 ωp ). The electron density cannot exceed the electrons available
in the ambient atmosphere, therefore the height of the emission is limited to approximately
the height below which the atmospheric atomic number density, natm , is equal to ne1 . Using the altitude-dependent atmospheric density from the “US Standard Atmosphere 1976”(USSA) atmospheric model2 , in a complete breakdown of the atmosphere we estimate themaximum available electron density at altitude h to be ne (h) ≃ natm (h) = ρ(h)NA /M (h),where NA is the Avogadro constant, and M (h) and ρ(h) are the mean atomic weight and2

The U. S. Standard Atmosphere.
NASA, Washington,
http://modelweb.gsfc.nasa.gov/atmos/us standard.html

D. C. (1976),

available from

130

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Datadensity of the atmosphere at altitude h.
We estimate the frequency-dependent delay traced by a flare propagating verticallywith velocity v by fitting the USSA atomic density profile withnatm (h) =[ρ(0)NA /M (0)] · exp(h/7 km)
4.7 × 1029 m3 (h/km)−6

for h ≤ 120 km(6.1)

for h > 120 km

The piecewise function accounts for the abrupt change in atmospheric makeup near thebase of the ionosphere. Based on our highest observing frequency and assuming we detected a second emission harmonic, we put an upper limit of 200 km as the limiting altitude
for such emission. For a detection of the first emission harmonic, the limit is more stringent(< 163 km).
Given a flare propagating along a transient plasma channel in which ne = k natm ,where k is the fractional ionization of the atmosphere, the frequency sweep exhibited byLangmuir-wave emission will depend on the velocity of the flare. Defining an upwards
normal vector, a flare emitting at f1 at an altitude h1 will later move to emit at f2
from altitude h2 , causing an observed altitude-dependent δt = t2 − t1 = (h2 − h1 )/v.

Determining h(f ) based on Eq. 1 and f (ne ), we obtain the frequency-dependent delay of
a flare of velocity v:
14 · ln[(k2 f1 )/(k1 f2 )]/v
δt =
(g/b)1/6 (√k /f )1/3 − (√k /f )−1/3 )/v
2 2
1 1

for h ≤ 120 km(6.2)

for h > 120 km

where g = 4.7 × 1029 m3 , b = 4me ǫ0 π 2 /e2 , and k1 , k2 correspond to the fractional at-mospheric ionization at h1 and h2 , respectively. A fit of this equation to the frequencydependent arrival times of Peryton 14 is shown in Fig. 6.4. This fit demonstrates that thefrequency-dependent sweep expected from a flare travelling upward through this channel
gives a high-to-low frequency sweep in rough agreement with the data, while deviations
from a smooth frequency/altitude-dependence might be explained by local density variations. Based on the observed total delays across the observing band, we find velocities
from 7 to 15 km s−1 if the atmosphere is completely ionized (k1 = k2 = 1). The clustering of band delays as shown in Fig. 6.2 indicate a characteristic velocity corresponding
to the peak of the histogram. The velocity value estimates will change if one or bothof the following applies to our simplified model: a) k is altitude-dependent, leading tosteeper ne gradients, or b) The channels of ionization (and thus progression of emission)
are non-vertical.

Difference

Relative arrival time (seconds)

6.4. Discussion

131

0.2
0.15
0.1
0.05
Dispersive delay
Vertical atmospheric propagation
Burst 14 band-dependent arrival times

0
0.01
0
-0.01
0.01
0
-0.01
1250

1300

1350

1400

1450

1500

Frequency (MHz)Figure 6.4 Frequency-dependent arrival time of Peryton 14 and models. The upper panel
shows the arrival time (points and errors) in 24-MHz-averaged bands relative to the 1512
MHz arrival. The best-fit dispersive delay is plotted in black and the Langmuir-emission
delay expected from a simple atmospheric model is plotted in gray. The lower panels
show the residual arrivals after subtraction of the models. To avoid preselection towards
a dispersion sweep, this timing analysis used sub-bands that were not dedispersed before
averaging, and employed the brightest sub-band as a timing template.

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Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data
If this mechanism is indeed the cause of the Perytons, the upward propagation, highplasma density, and >350 ms ion persistence time necessary to produce our signal suggest
an association with upper-atmospheric storm phenomena such as the aforementioned red
sprites, or gigantic jets (Sentman et al. 1995, Wescott et al. 1995, Su et al. 2003).

6.4.3

Weather-related Effects: Ducting, Annual Cycles, and Storm Activity

While Australian storm rates do not peak in June/July, this period coincides with Australian mid-winter. On the days of four independent detections for which data was available, the online Parkes weather archives3 indicate moderate rainfall on three, despite agenerally dry climate. This evidence leads us to suggest that such weather conditions
and the atmospheric state at mid-morning in winter provide an elevated likelihood forhigh-frequency tropospheric ducting modes to form (Hall 1979, Union 1996), allowing thedetection of distant emission not usually detectable by the telescope. Elevated detections during mid-winter mornings of transmitters known to be beyond the Parkes horizon
provide strong support for this hypothesis (provided via private communication by J.
Reynolds). Australian lightning activity was not elevated to a statistically significant level
for the days of detection 4 , however we do find cloud-to-ground lightning strikes indicating
electrically active oceanic storms occurring within 6 minutes of each event, at distancesless than 2000 km from the telescope. These coastal storms represent possible origins forLangmuir emission in the presence of a tropospheric duct.

6.5 A Closer Look at the Lorimer Burst
Returning now to the event that motivated this work, we would like to expand on therelationship of the LB to the Perytons. Whether the LB has the same origin as our detections remains a somewhat open question, these signals add serious doubt its extragalacticinterpretation. Several qualitative differences between the LB and the Perytons warrant
examination. The LB was clearly detected in only 3 of the 13 beams (6, 7, and D). It is
marginally detectable in other beams, most prominently in beams 5 and C, and apparent
in a stacked time series of the remaining beams. Based on observations of the inner sidelobe pattern of the multibeam by L. Staveley-Smith (private communication), we find aposition of the LB (consistent with both the relative detected flux levels in beams 6, 7, C,
3“Parkes Observatory Weather Statistics” (accessed 2009):
http://www.parkes.atnf.csiro.au/cgi-bin/monitoring/wstats.cgi
4Queried in 2009 through the “Lightning Incident Archive Search”: http://www.gpats.com.au

6.5. A Closer Look at the Lorimer Burst

133

D, and the non-detections in the other beams) at R.A. 19.44 ± 0.08, decl. −75.17 ± 0.08.

That is, the relative signal levels of the LB conform to those expected from a boresight
signal, in agreement with the same conclusion of Lorimer et al. (2007). Therefore, if the
LB is a Peryton, it appears to be the only detection for which the telescope was pointed
directly at the emitter. Consequently, although the LB did not exhibit the same deep
spectral signal modulation as the Perytons, these differences are well accounted for by themulti-path effects which we interpreted to arise from the horizon-based sidelobe detectionsof the Perytons (§6.4.1).
Note that because the beam and sidelobe shapes of the multibeam scale with frequency,
the offset of the burst from the center of beam 6 will induce a spectral steepening of
the source, causing the intrinsic spectrum to be flatter than that originally reported byLorimer et al. (2007) by αintrinsic = αobserved −αinduced (where S ∝ ν α ). The Staveley-Smithmeasurements were made at two frequencies, allowing quantification of this effect. Within
the error of our positional measurements, the induced spectral index is αinduced = −1+0.9−2.0 .

Therefore, at our estimate of position, the intrinsic spectral index is αintrinsic = −2.5−0.6,

where we have measured the observed spectrum in beam 6 to follow αobs = −2.6.

If the LB was caused by a sky-based object (e. g. an aircraft, satellite, or the Langmuir
wave emission proposed above), we might expect to detect some movement of the LB
across the field of the multibeam. We can limit the movement by noting that because
the signal saturated beam 6 for the entirety of its sweep, it is clear that the emitter
did not travel sufficiently far to cross a null in the sidelobes of beam 6. Therefore, we
limit any movement of the emitter to < 35 arcminutes, corresponding to a distance ofd = 0.015 h/sin(θ), where h is the emitter’s altitude and θ is the angle between thetelescope’s line of sight and the LB’s velocity vector. At 12 km (a typical aircraft/cloudheight), this corresponds to a distance and velocity of 180 m and 500 m/s, respectively,
if the LB is moving perpendicularly to the line of sight. This does not put rigorous
limits on aircraft movement, however does place bounds on the quantitative model in
6.4.2, requiring either non-vertical movement of the emitter, or the condition k2 < k1 , forthe model to hold for the LB. If an accurate measurement of the inner sidelobes of themutlibeam receiver and their frequency-dependence were available, it may be possible toplace a more stringent limit on (or provide a measurement of) movements of the LB.
One major point of discrepancy remains between the LB and the Perytons: that
their widths disagree by a factor of ∼2 (which cannot be accounted for by multi-path
scatter broadening), and we do not observe a frequency-dependent pulse width evolution

in the Perytons. However, we are hindered in measuring frequency-dependent evolution

134

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Datain the Perytons because of their modulated signal. While the difference in pulse width
lends weak remaining support for a divide between the LB and the Perytons, with ourcurrent measurements it cannot be ruled out that there may be an underlying pulse width
distribution, and/or a dependence on width with an event’s intrinsic flux; likewise wecannot state whether frequency dependence of pulse width is intrinsic to the phenomenon
causing all the events.

6.6 Conclusions: Implications for Current and Future Transient
Experiments
Regardless of the physical origin of the Perytons or the LB, the results of this study illustrate the limitations of single-dish radio burst detection experiments to provide conclusive
evidence for the origins of one-off bursts; dispersive delays can provide the only evidence
for an astrophysical nature on single-detector detections, while multi-detector (i. e. array)experiments can provide wavefront measurement and localization for localized pulses, or
can assure non-correlation of local signals between widely-spaced array elements. Arrayexperiments are necessary to provide a conclusive origin for further detections of the classof pulses presented here, as well as for any future experiments that aim to detect and use
extragalactic pulses in scientific studies. Two such experiments are currently underway at
the Giant Metre Wave and Very Long Baseline Array telescopes, and will be possible with
the Square Kilometre Array and its pathfinder experiments. Single-dish measurements
can improve our understanding of these events by providing polarisation measurements,and further detections for world, temporal, and delay-distribution statistics.
If our detections do originate from a natural terrestrial process with intrinsic flux≫100 Jy, detections are expected at other observatories with similar capabilities to Parkes

and ongoing pulsar and transient observations (e.g. Arecibo Observatory, Green Bankand Effelsberg Telescopes). It is an undeniable curiosity that no Perytons have yet been
identified at radio observatories where such searches have been performed (e. g. Amy et al.
1989, Nice 1999, Deneva et al. 2009). However, it is possible that they have not occurred
during observations (one should occur per ∼215 h of data on a telescope with similar systemtemperature and sidelobe suppression levels to Parkes, if the event rate is the same at othersites), that interference rejection algorithms based on the appearance of signals in multiple
beams have removed the signals, that the searches were at frequencies where Perytons are
not emitted, and/or that the experiments were not sensitive to pulses of 30-50 ms width.
As multi-detector coincidence filters will strongly preclude the detection of Perytons, the6.6. Conclusions

135

strength of such rejection filters to limit local interference of any form is clear; however
it is likewise clear that the filters would need to be relaxed to maximize an observatory’s
capability to detect and identify the origin of these remarkable signals. Additionally, we
believe that it was the human inspection of the spectrogram data and multi-beam timeseries for each candidate (described in §6.2) that were the main contributors to the firstrecognition of the peculiar nature of the Perytons—the simultaneous occurrence of clear,seemingly dispersed emission and multiple-beam detection—and that encouraged us to
scrutinize these events more closely instead of disregarding them as spurious detections.

136

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data

Detection #01

Detection #02

Detection #03

Detection #04

Detection #05

Detection #06

Figure 6.5 Spectrograms of all detections, as in Fig. 6.1(b).

6.6. Conclusions

137

Detection #07

Detection #07a

Detection #08

Detection #09

Detection #10

Detection #11

Figure 6.5 continued.

138

Chapter 6. Anomalous, Terrestrial Lorimer-Burst Signals in Pulsar Data

Detection #12

Detection #13

Detection #14

Detection #15

Figure 6.5 continued.

7Conclusions and Future Directions

7.1 Conclusions and Major Findings of this Thesis
Motivated by the broad-reaching scientific endeavours encompassed by “pulsar astronomy,” this research has advanced the knowledge of various astronomical and physical
topics. In particular:
Gravitational Wave Astronomy: In Chapter 2, we examined the possibility of detecting supermassive black hole binary systems through both gravitational wave and electromagnetic signatures. We concluded that two electromagnetic tracers of black holes—highresolution radio spectral mapping and emission lines from the gas surrounding an active
galactic nucleus—have significant overlap in the masses, orbital periods, and distances ofsystems to which pulsar timing is theoretically sensitive to their gravitational radiation.
Blind searches for electromagnetic signatures of binary supermassive black holes will belimited by the number of electromagnetically-emitting cores in the Universe, but discoveries should be possible given a rigorous search of a large sample. The identification of binary
systems emitting gravitational waves in the pulsar timing band will raise the sensitivity of
pulsar timing arrays manyfold and may enable the first direct detection of gravitational
wave emission as pulsar timing improves its sensitivity to gravitational waves from theseobjects.
Galaxy merger dynamics: Exploiting one of the search methods of Chap. 2, we embarked on a search for supermassive binary systems in archival radio data. The possibility
that supermassive binaries might “stall” at small orbital separations has been an ongoingissue in post-merger dynamical theory that—if more than theoretical fancy—would havedetrimental implications for detecting gravitational radiation with pulsar timing arrays.
Our search resulted in the first observational evidence against this possibility. The data
indicate an efficient inspiral evolution to within the purported stalling radius, however
139

140

Chapter 7. Conclusions and Future Directions

with a timescale of sufficient length that it may produce a non-negligible effect in predictive models for the amplitude of the gravitational wave background in the pulsar timing
band.
Radio transient searches and search methods: We have developed a new inspection
and anti-coincidence search methodology that uses the methods outlined by Cordes &McLaughlin (2003), and expands on them by employing several novel techniques: 1) identification of discrete events and reconstruction of their observed properties, 2) automateddispersion measure- and multi-detector-based interference excision, and 3) candidate inspection of data for single events. These techniques have enabled improved discrimination
between interference and low-dispersion (i. e. very nearby, or high galactic latitude) impulses, as evidenced by the low dispersion measure of many of our discoveries (19 below
DM = 88 pc cm−3 , with the lowest having a dispersion measure of DM = 9.2 pc cm−3 )as compared to the original search that discovered “rotating radio transients” (RRATs
McLaughlin et al. 2006; all above DM > 88 pc cm−3 ). The software to perform thesesearches is highly portable and has been made available to the public domain for use atother radio telescope facilities.
Neutron star discoveries: Utilising the search methods and software developed inthis thesis, we have contributed the discovery of 27 neutron stars to the known pulsar
population. Approximately 90% of the new discoveries represent a significant contribution
to extreme nulling pulsars, of which previously .50 were known. Of particular interest
are several objects. PSR J0941–39 appears part-time as a low-fraction nulling pulsar andotherwise appears as a sparsely-emitting “rotating radio transient,” and likely represents
an evolutionary transition between two states of emission. PSR J0410–3107 holds several“records” for extreme nulling pulsars/RRATs, being the closest known RRAT to Earth,that of the highest galactic latitude, and lowest dispersion measure (9.2 pc cm−3 ; this DM
is also lower than 98.9% of known radio pulsars).
Insights into sparsely emitting neutron stars: The new discoveries of sparselyemitting neutron stars have revealed further insight into the nature of the pulsar-related
phenomenon “rotating radio transients.” We have concluded that some of the objects
classified as RRATs do appear to be genuinely intermittent (rather than faint/distantpulsars with long-tailed pulse energy distributions). Furthermore, the RRAT population
appears to have both a galactic distribution and a duty cycle distribution very similar to
average pulsars. These insights, as well as the pulsar J0941–39’s bimodal emission states,
support the suggestion that RRATs are more likely to be extreme examples of nulling
pulsars, where the length of their “on” emission is less than one rotational period. The7.2. Future directions

141

analysis of emission and null timescales for transient emitters suggests that rather than
RRATs, the “intermittent pulsars” like PSR B1931+24 (Kramer et al. 2006) appear torepresent a distinct class of transient neutron star phenomena.
The dynamic radio sky: We have made several new neutron star discoveries in theHigh Time Resolution Universe Survey in regions of overlap with previous pulsar andtransients surveys. These new discoveries are a testimony to the dynamic nature of thetransient radio sky, and the analysis in Chapter 5 has indicated that surveys of longpointing duration, e. g. the HTRU low-latitude survey, will be capable of both discovering
pulsars with nulling fractions greater than 99.99% (that is, pulsing only once per 3-4
hours), and will explore what appears to be a deficit of neutron stars which are active fortimescales above 300 s but have nulling lengths greater than ∼100 s.
Terrestrial and extragalactic radio transient phenomena: We discovered 16 impulses (“Perytons”) of .50 ms duration whose swept-frequency signal closely mimics, however is not caused by, cold plasma dispersion. The pulses are caused by an unknown
phenomenon of terrestrial or Earth-local origin. Their existence has several consequences:
1) They draw significant doubt on the origin of the only reported detection of an extragalactic radio impulse (Lorimer et al. 2007); 2) Their similarity to cold plasma dispersion
severely limits the use of many pulsar surveys to characterise extragalactic pulse populations. Specifically, the single-detector experiments in current common use for pulsar
searches have insufficient capability to definitively distinguish one-off terrestrial pulses
from those of celestial origin; 3) There is an unidentified terrestrial phenomenon which
exhibits strongly frequency-swept emission. We have explored a possible atmosphericplasma origin with a model that can reproduce most of the properties of the pulses. If
this is the source, it may imply possible ties to the upper-atmospheric Transient Luminous Event phenomena (e. g. Su et al. 2003) that on occasion accompany electrical storm
activity. However, further exploration and further detections are undoubtedly required todetermine the actual pulse origins.

7.2 Future directions
Pulsar astronomy remains an ambitious and far-reaching scientific field. This thesis has
contributed to two areas, radio transient and gravitational wave astronomy, as their own
rising and highly dynamic fields of research. The work in this thesis has opened a base of
new questions that may be answered using both current radio telescopes and the upcoming
generation of telescopes such as the Square Kilometre Array and its pathfinders. Here we

142

Chapter 7. Conclusions and Future Directions

provide the directions for possible future work that are motivated or strengthened by our
results.
1. The discovery of significant numbers of supermassive black hole binaries. We haveshown the power of SMBH binary searches to limit pair evolution dynamics, and indicated that electromagnetic signatures of gravitationally-emitting SMBH binaries
should exist. There is yet one system with solid evidence for a binary nature, andfurther discoveries will enable a deeper understanding of post-merger galaxy dynamics and the expected gravitational signatures for pulsar timing. We have shown thatradio-detectable binary systems are uncommon (in our sample, they represented only
0.03% of the radio AGN we searched). However, our discovery rate was limited not
only by the paucity of binary systems, but also by our low mean observing frequency(resulting in a coarse resolution limit) and the shallow flux sensitivity of our data.
Using radio search techniques, numerous discoveries should result from employing
a sample of targets with a-priori knowledge of a probable recent merger, and/or
observations that reach spatial resolutions down to .a few pc for all targets at aflux sensitivity deep enough to raise the probability of detecting two radio-emitting
black holes in a system to one (i. e. if both black holes in a common environment
are equally radio luminous, observe deep enough that both objects are detectable
regardless of their jet angle). The Square Kilometre Array will represent an advancement in its capability to quickly survey large numbers of radio galactic nuclei to highsensitivity. Offset emission-line identifiers have revealed a large number of binary
candidates, but a definitive origin of the peculiar spectra has yet be determined.
Radio imaging may contribute to the resolution of this debate in the future.
2. Are RRATs special?

The large number of detectors currently capable of short-

duration pulse detection (e. g. on Green Bank Telescope, Arecibo Telescope, andthe continuing HTRU survey at Parkes) will contribute to a large increase in known
sparsely-emitting neutron stars. The discovery of new objects, and the determination of the single-pulsing population’s properties are two essential elements indetermining, for instance, if many other sources exhibit the same bimodal emissionof our discovery PSR J0941–39. Currently ∼50 objects strong, the known RRATs
are beginning to form a strong statistical sample for comparison of properties against

other neutron star populations, e. g. pulse energy distributions and luminosity functions. Determining timing solutions of these objects will allow more fundamental
properties, such as magnetic field strength and the age, to be used in deriving correlations between such properties and the objects’ emissivity. An interesting alternative

7.2. Future directions

143

to phase-connected timing solutions may be warranted, however, given the time involved in acquiring a timing solution. If an interferometric system had the capabilityof detecting and imaging single pulses, the positions of RRATs could be accurately
determined. This would provide several benefits; first, accurate positions can be used
to pinpoint RRAT emission at high-energy (X- and γ-ray), and second, an accurate
position would break the timing degeneracy between position and period derivative,
allowing the estimation of an RRAT’s magnetic field strength, age, etc. based only
on the interferometric observation and a few timing sessions spaced at large timeintervals. Such a system would need to be in place to make efficient use of telescope
resources when determining the properties of large quantities of RRATs.
3. Nulling and modulation in radio pulsars Our new discoveries have supported thepossibility that the sources observed as RRATs may be a mix of both high nulling
fraction pulsars and highly modulated pulsars with long pulse energy distribution
tails. Thus far, modulation studies have typically focussed on a small number ofpulsars, and no statistical nulling studies have yet incorporated the radio pulsars
with nulling fraction ≫95% that have been discovered only since the return of single-

pulse searches. Our conclusions on RRAT and nulling phenomena may be supported
or negated with rigorous studies of the single pulse energy distributions of brightradio pulsars over a range of characteristic age, magnetic field strength, spin-downluminosity, and nulling fraction.
4. What is the origin of Perytons? One of the major discoveries made in this thesis
was of the terrestrial radio impulses that mimic the frequency sweep of cold plasma
dispersion in the interstellar medium. While we have presented the possibility ofan atmospheric origin for these pulses, evidence for a conclusive origin is yet to becollected. Transient detection experiments on localised multi-detector instruments,
e. g. those currently operational on the Giant Metre-wave Radio Telescope and the
Allen Telescope Array, and that are planned for the Australian Square Kilometre
Array Pathfinder will need to consider our findings. These instruments are expected
to detect many examples of Perytons. The use of multiple detectors to measure thewavefront of incident events will enable a precise localisation of future terrestrial
burst detections, hopefully revealing a clear picture of the man-made or naturalcharacter of the pulses.
5. The timescales of radio pulsar activity cycles The large field of view and high sensitivity of the upcoming Square Kilometre Array and its pathfinders provide the op-

144

Chapter 7. Conclusions and Future Directions
portunity to perform dedicated and/or piggybacked fast-transient imaging surveys
over a large sky area. These surveys have the potential to discover neutron stars
undergoing activity cycles that last from 5 seconds to several hours, to several days.
However, there will be significant challenges in identifying the underlying natureof transients discovered by these surveys, and non-pulsed observational markers forpulsars (e. g. polarisation) will need to be identified to understand which transientphenomena may be attributed to intermittent neutron stars.

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A
Appendix A: Useful constants and conversions

For reference and convenience, here we list the constants and common conversions usedthroughout this thesis.
Avogadro’s constantNA=

Boltzmann constantkB=

Dispersion constant

D

=

e

=

Gravitational constantG=

Hubble constant

H0=

Hubble parameterh0=

Hubble timetH=

Mass of electronme=Permittivity of free space

ǫ0

=

Pi

π

=

Speed of lightc=

Solar mass

M⊙=

Elementary charge

1 pc

=

1 year

=1 Jansky=

6.02214 × 1023 mol−1−11.38065 × 10−23 m2 kg s−2 K

4.14881 × 103 MHz2 pc−1 cm3 s
1.60218 × 10−19 C

6.67300 × 10−11 m3 kg−1 s−2100 h0 km/s/Mpc = 2.3334 × 10−18 Hz
0.72 (unless otherwise stated)

13.58 × 109 years = 4.286 × 1017 s
9.10938 × 10−31 kg

8.85419 × 10−12 m−3 kg−1 s4 A2
3.14159265358979

2.99792 × 108 ms−1
1.98892 × 1030 kg

3.08568 × 1016 m

3.15569 × 107 seconds

10−26 W m−2 Hz−1

157