Monitoring X-ray Binary Pulsars with Fermi/GBM
P. A. Jenke1 and C. A. Wilson-Hodge2
1
2

University of Alabama in Huntsville, Huntsville, AL 35812, USA
NASA, Marshall Space Flight Center, Huntsville, AL 35812, USA
E-mail(PJ): peter.a.jenke@nasa.gov
Abstract

The Fermi Gamma ray Burst Monitor (GBM) is a unique instrument that oﬀers the largest instantaneous field of view of any hard X-ray instrument currently in operation. This capability along withexcellent timing resolution, makes it very successful at detecting rare transient events as well as providing long integration times for pulsed signal extraction. Even though GBM has a relatively modest size,
we are able to observe a typical accreting pulsar for over 40,000 seconds each day allowing us to makeprecise measurements of the source frequency and pulsed flux for sources with a spin frequency between
0.001 and 2 Hz. These frequency measurements due to GBM’s excellent timing capabilities have given us
the capability to determine/update orbital ephemerides for many sources as well as monitor rare torquereversals in persistent (semi-persistent) sources such as EXO 2030+375. Continuous Time Tagged Eventdata, available since November 2012, allows the GBM pulsar monitor to track the frequency of even higher
frequency sources and we plan to make these histories available this year.
Key words: pulsars,neutron stars

1. Introduction
Using the Gamma ray Burst Monitor (GBM), the GBM
Pulsar Monitoring program (GPM) is designed to monitor the frequency and pulsed flux of accreting pulsars
and detect new outbursts from transient pulsars withspin periods between 0.5 to 1000 seconds and in the energy range between 8 and 50 keV. GBM is not an imaging
instrument but has the advantage of observing the entire
unocculted sky at all times with the exception of brieftransverses of the South Atlantic Anomaly. This fullsky coverage enables long term monitoring of accreting
pulsars, allowing precise measurements of spin frequencies, pulsed flux, orbital parameters enabling studies ofthe flow of angular momentum from the donor star onto
the neutron star and constraints on the magnetic field.
The long source exposures possible with GBM are crucial for precise timing measurements, where the error on
pulse frequency and pulse frequency rate estimates scaleas T−3/2 and T−5/2 , respectively, where T is the observation duration.
The pulsar monitor has a sister program, the GBM
Earth Occultation Monitor (GEOM), which uses the fall
and rise of the count rate of known sources as they are occulted by the Earth’s limb and reappear again (WilsonHodge et al. 2012). This program is currently monitoring over 200 sources and is able to provide flux histories
for these sources in the energy range between 8 and 300

keV. A spin-oﬀ program, the X-ray Burst (XRB) monitor uses the pulsar monitor’s data cleaning process to
identify thermonuclear type 1 XRBs from low Ṁ accreting neutron stars and has identified 752 photosphericradius expansion (PRE) burst in its first three years ofoperation starting in March 2010 (Jenke et al. 2016).
1.1. Gamma Ray Burst Monitor
GBM consists of 12 NaI detectors with a diameter of
12.7 cm and a thickness of 1.27 cm and two BGO detectors with a diameter and thickness of 12.7 cm. The NaI
detectors have an energy range from 8 keV to 1 MeV,
while the BGOs extend the energy range to 40 MeV.
There are three continuous publicly available data types:
CTIME data which has 8 energy channels (8–1000 keV
for NaIs) with 256 ms timing resolution and is primarily used for localization and long event searches, CSPEC
data which has 128 channels (8–1000 keV for NaIs) with
4,096 s timing resolution and is primarily used for spectroscopy, and CTTE data which has 128 channels (8–1000 keV for NaIs) with time tagged photon events with
2 µs precision and is used for spectroscopy, timing, andshort interval searches. The pulsar monitor primarily
uses CTIME data but is expanding its capability withCTTE data which increases sensitivity for higher frequency pulsars and improves pulsed spectral resolution
for bright sources.

2. GBM Pulsar Monitor
The pulsar monitor consists of two eﬀorts: (1) The daily
blind search aims at detecting new pulsed transients andoutbursts from known transients. (2) The dedicatedsearch is currently monitoring the pulsed flux and frequency of 39 sources and has pulsed flux and frequencies histories for 36 detected sources. GPM is providing,through its website1 , quick-look estimates of pulsed flux
and frequency for use in multi-wavelength observations.
The website is updated twice a week and announcementsof new outbursts from transient sources are published in
Astronomer’s Telegrams to facilitate follow-up observations.
2.1. Pulsed Searches with GBM
The analysis of GBM observations of pulsars presents
two main challenges: the background rates are normallymuch larger than the source rates, and have large variations; and the responses of the detectors to a source arecontinuously changing because of Fermi’s ever changing
orientation. GPM meets these challenges with an empirical background subtraction and extensive on the fly
calculations of detector response matrices. The initial
step in our analysis consists of screening the NaI detector count rate data to remove phosphorescence events,
gamma-ray burst and particle events and intervals of
rapid spacecraft rotation. Then an empirical backgroundmodel is fit to the rates and subtracted oﬀ, after which
the rates are combined over detectors in a way that results in an estimate of the variable part of the source flux.
From these flux variation estimates we conduct searches
for pulsations, and monitor detected sources.
2.2. Empirical Background Subtraction
The screened rates in each channel of the 12 NaI detectors are fit with a model with that includes components
for a small number of bright sources and a stiﬀ empirical model that contains the low frequency component of
the remaining rates. The source models are computedfrom a model flux spectrum using time dependent response matrices. One fit is made for each channel with
the normalization factors for the bright sources common
to all detectors, but with the stiﬀ model parameters (astatistically constrained spline) independent for each detector. The model accounts for the occultation steps forthe bright sources and the variation of the backgrounddue to spacecraft motion while upon subtraction retains
the higher frequency signals of the frequency modulated
sources.
2.3. Daily Blind Search
Using the background subtracted data, time dependent
responses and a generic pulsar spectrum (power law with
*1 https://gammaray.msfc.nasa.gov/gbm/science/pulsars.html

a high energy cut-oﬀ), source fluxes (8–50 keV) are determined for 26 directions along the Galactic plane (15degree cadence) plus the LMC and SMC. The source
fluxes are Fourier transformed and searched for significant pulsations. The peak frequencies and directions are
determined by interpolating over the two dimensional
grid of Fourier power and directions. A catalog of accreting pulsars with the last known spin frequency is compared with the peak frequency and direction in order to
identify the pulsed signal. Significant unidentified pulsed
signals are flagged for additional analysis. Additional
analysis is limited by GBM’s large location uncertainty(a few degrees) but typically includes searching for significant pulsations from a grid of directions that include
higher latitudes and searching for new flaring pulsars in
the MAXI X-ray monitor (Matsuoka. et al. 2009) and
Swift BAT Hard X-ray Transient Monitor (Krimm et al.
2013).

2.4. Dedicated Search
In the dedicated search, source fluxes (8–50 keV) aresimilarly determined for the specific monitored pulsar
position and energy spectrum of the source of interest.
Appropriate lengths of data (usually a minimum of 5–
10 times the pulse period) are fit to a Fourier expansionin pulse phase using a course phase model. Times arebarycentered using the JPL Planetaria ephemeris DE200(Standish 1990) and, if an ephemeris is available, corrected for orbital motion. Fourier components for shortintervals (suitable for the source but usually a few days)
are combined and searched for significant pulsations in
up to three harmonics over a small range of frequency
and possibly frequency derivative. This provides higher
sensitivity than the blind searches because of the smaller
search ranges and the use of source specific informationsuch as the location, orbital parameters and flux spectrum.
GBM pulsar monitor currently has frequency histories and pulsed fluxes for 36 accreting binaries including 28 transients. Figure 2 lists the currently detectedtransients, most of which are Be X-ray binaries, andthe times which GBM detects them. Of particular interest is the recent outburst of SMC X-3 in July 2016(see Figure 1), currently the only SMC source GBM detects. EXO 2030+375 is GBM’s most persistent transient source. Until recently, the GBM pulsar monitor
has detected an outburst almost every periastron passage. The Be X-ray binary’s spin-up flattened and thesource faded from detection for 5 orbital periods in April
2016. When it reemerged on MJD 57632, the source had
undergone a rare torque reversal. Surprisingly, after two
more orbits, EXO 2030+375 appears to have resumed its
previous spin-up state (see Figure 3).

GRO J1744-28
4U 1901+03
4U 0115+634
V 0332+53
GRO J1750-27
SMC X-3
2S 1553-542
XTE J1859+083
RX J0520.5-6932
IGR J18179-1621
GS 0834-430
IGR J19294+1816
XTE J1946+274
2S 1417-624
KS 1947+300
Swift J0513.4-65
EXO 2030+375
Cep X-4
GRO J1008-57
2S 1845-024
A 0535+26
MXB 0656-072
RX J0440.9+4431
XTE J1858-034
GX 304-1
SAX J2103.5+4545
A 1118-616
MAXI J1409-619

SMC X-3Spin Frequency (mHz)

128.6
128.5
128.4
128.3
128.2
128.1

0.4
(keV cm-2 s-1)

12-50 keV Pulsed Flux

128.0

0.3
0.2
0.1

55000

55500

0.0
57660

56000
56500
Time (MJD)

57000

57500

57680

Fig. 1. The top panel is the frequency history of SMC X-3 while
the bottom panel is the 12–50 keV pulsed flux. The frequencies
have been corrected for orbital motion using the ephemeris from
Townsend et al. (2017). Arrows indicate upper limits and the
diagonal bars through the frequency points represent the best
frequency derivative.

2.5. Orbital Ephemeris Determination
Typically, after correcting the pulse arrival times for
Earth’s motion, one can use the doppler boosted frequencies of the pulses to fit a model for the binary system fora source of interest. Material accreted onto a neutronstar surface or collected in an accretion disk threaded by
the neutron star’s magnetic field transfers angular momentum to the neutron star. Disentangling this intrinsic
spin-up from the orbital signature is challenging. A solution to this problem is to model the intrinsic spin-up
using a proxy for the system’s X-ray luminosity. The
luminosity is a function of mass accretion which is related to the torque imposed on the neutron star. Rappaport & Joss (1977) showed that, at high luminosity,
the intrinsic spin-up, ν̇, is proportional to L6/7 whenaccretion is mediated through a disk. The proportionality constant is a function of mass, radius, moment ofinertia, distance and magnetic field of the neutron staralong with a few parameters describing accretion andemission eﬃciency. When connecting multiple outbursts
with the same torque model, it is necessary to include a
spin-down term to account for angular momentum losses
during quiescence (see Figure 4).
A search for frequency and frequency rate is performed
using pulse profiles from GBM folded over a multi–day
interval. Each frequency epoch is chosen as the mean
exposure–weighted observation time. The epochs arebarycentered using the JPL Planetary ephemeris DE200(Standish 1990). BAT survey data (15–50 keV) or MAXI
X-ray data (10–20 keV) for the individual source is used
as a proxy for the source luminosity and to model theFig. 2. The GBM detected transients are listed vertically while thetimes where they are detected are indicated by the blue bars.
EXO 2030+375
Spin Frequency (mHz)57640
Time (MJD)

24.22

24.21

24.20

24.19
0.5
(keV cm-2 s-1)

57620

12-50 keV Pulsed Flux

57600

0.4
0.3
0.2
0.1
0.0
55000

55500

56000
56500
Time (MJD)

57000

57500

Fig. 3. The top panel is the frequency history of EXO 2030+375 while
the bottom panel is the 12–50 keV pulsed flux. Arrows indicate
upper limits. Notice the torque reversal and recovery around MJD
57300.

intrinsic spin-up rate. This spin-up model along with theline of sight delay associated with the binary orbit from
Deeter et al. (2003) is used to model the barycentered arrival times. Minimization of the χ2 fit of the barycentricfrequencies and the count rates (Swift/BAT or MAXI)
is performed using the Levenberg-Marquart method andis of the form:∑2χ2 =; i = 0, n − 1(1)
i (fi − (ν˙i (1 − βi ))∑2+ i (Xi − ν˙i /m); i = 0, n − 2(2)• βi is the orbital redshift factor at time ti which is afunction of the orbital elements.
• fi is the measured barycentric frequency at time ti
which is the frequency epoch of the search interval

4U 0115+634
276.694
Spin Frequency (mHz)

276.692276.690276.688276.686276.684

Spin-down during quiescence

1.0(keV cm-2 s-1)

8-25 keV Pulsed Flux

276.682
1.2

0.8

October 2015

0.6

June 2011

0.4
0.2
0.0
55000

55500

56000
56500
Time (MJD)

57000

57500

Fig. 4. The top panel is the frequency history of 4U 0115+634 while
the bottom panel is the 8–25 keV pulsed flux. The frequencies
have been corrected for orbital motion. There are two giant outbursts in June 2011 and October 2015 where there is significant
spin-up do to accretion torques. After the giant outbursts, thepulsar spins down until a regular outburst occurs and there isagain spin-up do to accretion torques although the spin-up is not
as severe. Even between the normal outbursts there is significant spin-down (red arrows) which is modeled as 1 or 2 degree
polynomial when fitting the frequencies to an orbital model.

i. Each epoch, ti , is chosen as the mean exposureweighted observation time.
• Xi is the average value of R6/7 between ti and ti+1
and R is the MAXI count rate.
• The model parameter νi is the orbitally corrected
frequency at time ti and ν˙i is (νi+1 − νi )/(ti+1 − ti ).
The updated orbital model is used in a new search for frequencies and frequency rates recursively until the orbital
solution becomes stationary. Figure 5 demonstrates thistechnique. Notice how well the torque model follows thetransition between a normal and giant outburst around
MJD 56200.
2.5.1. Long Term Monitoring
Combining GBM data with data from the pulsar monitor’s predecessor which used data from the Burst andTransient Source Experiment (BATSE) on board theCompton Gamma Ray Observatory (CGRO), long term
pulsed histories spanning twenty years may be createdalthough there is a significant time gap between CGRO
and the Fermi mission. Searches for orbital decay are
possible by determining the orbital epoch at intervals
over a long period of time. This is important for understanding how diﬀerent X-ray binary populations might

Fig. 5. Black points are the barycentered frequencies for GX 304−1.
The black curve is the best fit torque model from the Swift/BAT
rates while the red curve is the orbital model combined with thetorque model. Notice how well the torque model represents thetorque during the onset of the giant outburst at MJD 56200.

be related through evolution. Orbital solutions are determined where the source of interest was bright for theavailable BATSE and GBM data. The Tπ/2 epoch is
expanded in orbit number, n,1
Tπ/2 = To + nPorb + n2 Porb Ṗorb ,2(3)

and the quadratic term determines the orbital decayof the source. The expected linear trend in the orbital epoch is subtracted and the residuals are fit toa quadratic polynomial corresponding to the last termin Eq 3 (See Figure 6). Significant orbital decay (Ṗ =(−9.74 ± 0.77) × 10−8 ) was detected in the wind fed highmass X-ray binary OAO 1657−414 (Jenke et al. 2012).
3. GBM Earth Occultation Monitor
The Earth occultation technique (EOT) measures thechange in flux when a source enters (or exits) Earthoccultation (see Figure 7). The occultation time must
be known so a catalog of sources is used and updated
regularly. The changing GBM detector response, atmospheric transmission, the source spectrum, and othersources also occulting within a four minute fit window(including the flaring Sun) are taken into account. The
EOT is the only technique currently available that enables GBM to monitor persistent and transient galacticand extragalactic sources on timescales of days to years.
GEOM is currently monitoring 247 sources and light
curves (12–300 keV in four energy bands) and fits files(12–1000 keV in six energy bands) are available on

Fig. 6. Diamonds are orbital epochs for each ephemeris with a linear
trend removed. The curve is a quadratic fit to the epochs. The
residual epochs to the left are calculated from BATSE data while
the residual epochs on the right are calculated from GBM data.

Fig. 8. GEOM results monitoring the flux for Cygnus X-1 shown for
12–25, 25–50, 40–100, 100–300, 300–500 keV energy bands. The
vertical colored lines indicate times of state changes.

4. GBM XRB Monitor

Fig. 7. Example of an occultation fit window. The occultation stepfor the source is clearly seen at t=0. The best model for the fit
window is shown by the solid curve.

the web site2 . GEOM has confidently detected over
100 of its monitored sources with 9 sources being detected (> 5σ) above 100 keV (Case et al. 2011). The
sources detected above 100 keV include 7 persistent and
2 transient sources: the Crab, the BHs Cyg X-1, Swift
J1753.5−0127, GRS 1915+105, 1E 1740−29, and GRS
1758−258, the AGN Cen A, and the transient BHs XTE
J1752−223 and GX 339−4. We have also detected brief,bright events with single Earth occultations including
numerous solar flares, and a brief episode of enhanced
persistent emission from SGR 1550−5418.
GEOM compliments the GPM by monitoring transient outbursts of Galactic accreting binaries and measure the total flux of the source. GEOM can also monitor
spectral changes in these systems which may accompany
a change in the accretion state especially in bright blackhole binaries such as Cyg X-1 (see Figure 8).
*2 https://gammaray.msfc.nasa.gov/gbm/science/earth occ.html

Bright transient events are removed from the data prior
to fitting the background in preparation for the pulsar
search. These events were cataloged for further analysis
under the GBM XRB Monitor. The events are localized by using the angular response of the NaI detectors
to reconstruct the most likely arrival direction based on
the diﬀerence in the background subtracted count rates
in the 12 NaI detectors which have diﬀerent sky orientations. The method is adapted from the method used for
GBM GRB localization (Connaughton et al. 2015). The
XRB localization uses the 12–50 keV rates and spectral
models appropriate for sources with softer energy spectra. Three cases are automatically identified: If the event
localization is consistent with being toward the Sun thenthe event is identified as being solar, If the orientationof the bright detectors are inconsistent with a single direction then the event is identified as a particle event.
Particle events tend to produce signal in all the detectors
or in detectors with opposite orientation. If the event localization is consistent with a single direction and is not
associated with the Sun then the event is considered acandidate XRB. These candidates undergo further analysis such as a refined localization and spectral analysis
which occasionally resulted in a reclassification and rejection of the candidate status reducing the final number
of XRBs to 1084. Using spectral analysis, location andspatial distributions we classified the 1084 events into

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Fig. 9. The red and green filled circles are the know type 1 XRBs.
The green filled circles are the XRBs that are suﬃciently isolated
from currently active sources that an association is possible. The
blue circles are the GBM detected events. The radius of the circle
is the approximate 1 sigma error region of the localization.

752 thermonuclear X-ray bursts (33 associated with 4U
0614+09), 267 transient events from accretion flares and
X-ray pulses, and 65 untriggered gamma-ray bursts. See
Figure 9 for the distribution of detected type 1 XRBs.
All thermonuclear bursts have peak blackbody temperatures broadly consistent with photospheric radius expansion (PRE) bursts (Linares et al. 2012). We find an
average rate of 1.4 PRE bursts per day, integrated overall Galactic bursters within about 10 kpc. These include
33 and 10 bursts from the ultra-compact X-ray binaries
4U 0614+09 and 2S 0918−549, respectively.