Precision Radio-Frequency Pulsar Timing &amp; Interstellar Scintillometry
Creators: name: Reardon, Daniel John
Publisher: name: Zenodo
Year: 2018
Subjects: Pulsar
Pulsar timing
Interstellar scintillation
Radio astronomy
Millisecond pulsars
Relativistic binary
Scintillometry
Pulsar scintillation

description: Pulsars are natural clocks that can be used as laboratories for strong-field gravity, and as sensitive probes of the interstellar plasma along our line-of-sight. Pulsar timing arrays (PTAs) monitor a set of the most stable millisecond pulsars (MSPs) over many years to function as a Galactic-scale gravitational wave (GW) detector, sensitive to nanohertz-frequency waves. The eventual detection of these GWs will rely on a detailed understanding of the pulsars and of the various sources of noise, including the interstellar plasma. In this thesis we improve the ephemerides for a set of these MSPs, and a relativistic binary pulsar, using the techniques of pulsar timing and scintillometry.

For each of the radio MSPs in the Parkes PTA, we have analysed their intrinsic spin noise and modelled small changes to the electron density along our line-of-sight. These are strong sources of non-stationary noise for pulsar timing, which we accounted for by using a new algorithm. We produced new timing models for each pulsar, resulting in the first parallax measurements for five pulsars and improvements to other parameters, such as the Shapiro delay to give new pulsar mass measurements. Our new distance measurement for PSR J0437-4715, \(D = 156.79 \pm 0.25\) pc, is the most precise for any pulsar. For the first time, the uncertainty on this measurement is less than the typical wavelengths of GWs that PTAs are sensitive to, which will increase sensitivity to continuous GW sources.

Scintillation of radio pulsars is results from scattering by the turbulent interstellar plasma. The scattering leads to a time-variable interference pattern that is sampled by the observatory moving through space along the Earth’s orbit. We use measurements of this scintillation to model the motion of the line-of-sight to the pulsar, which depends on the relative transverse motions of the Earth, pulsar, and scattering plasma. We have measured long-term changes in the diffractive scintillation pattern of a relativistic binary pulsar, PSR J1141-6545, and used it to determine several previously unknown important parameters for the system. In particular, we have measured the orientation of the orbit in celestial coordinates, \(\Omega = 23 \pm 3^\circ\) (East of North). We use our scattering model in a new way to estimate the pulsar distance \(D = 9^{+5}_{-3}\) kpc, and to give the first estimate of its proper motion in right ascension \(\mu_\alpha = 2.5 \pm 1.2\) mas/yr and in declination \(\mu_\delta = 1.5 \pm 0.7\) mas/yr. With these measurements, we constrain the kinematic contamination to the relativistic orbital period-derivative for this system.

Finally, we have capitalised on the amazing long-term data collected by the PPTA collaboration for PSR J0437-4715. The intensity measurements reveal parabolic "scintillation arcs" on ~1300 observing epochs, which vary in curvature with the changing line-of-sight velocity. By modelling these arcs, we have determined some orbital parameters of the pulsar, and properties of the interstellar scattering medium, with unprecedented precision. This method is a promising alternative to the more common diffractive scintillation modelling for pulsars that have well- defined scintillation arcs or that are observed in the regime of weak scattering.