High-Precision Pulsar Timing

Abstract

Pulsar timing is a technique that exploits the high rotational stability of pulsars, by comparing the average times of arrival (TOAs) of observed pulses with the predictions from a timing model. One of the premier goals in the field of pulsar timing is the direct detection of the stochastic gravitational wave background, through its influence on the TOAs from an ensemble of pulsars. The influence of gravitational waves on pulsar timing data is expected to be very small, and for a detection to be achieved, parameters intrinsic to the pulsar as well as extrinsic factors arising from effects such as propagation through the variable interstellar medium (ISM) must be precisely measured and accounted for. In this thesis, I consider the influence of scattering and dispersion measure (DM) variations, neutron star rotational instabilities, and pulse emission properties on precision pulsar timing measurements. Using 30 years of observations of the Crab Pulsar with Jodrell Bank Observatory's 42-ft telescope at 610 MHz, we have measured the temporal variations in scattering time scales from the influence on observed pulse shapes. Supplementing our 610-MHz data with data taken at 1410 MHz with the Lovell Telescope, we have used multi-frequency TOAs to make precise measurements of the DM variations over a 6-year period. We find that the DM and scattering time scales track each other very closely over this time, with a correlation coefficient of ~0.6. We attribute the variations in DM to discrete high-density regions within the nebula that are of a smaller scale than the 'wisps' seen in optical observations of the nebula, and given the short time scales over which the scattering time scales and DM vary, we estimate the size of these regions to be ~6 AU (equivalent to an angular size of ~2 mas at a distance of 2200 pc). We demonstrate a method of removing the influence of scattering from archival data, by using epoch-specific reference templates that take into account the measured scattering time scales. We find that accounting for the variable scattering time scales leads to a factor of two improvement in root mean square (RMS) timing residuals for the Crab Pulsar. We use TOAs from the European Pulsar Timing Array (EPTA) data release 1.0 (DR1), together with earlier TOAs from the Effelsberg and Lovell Telescopes to show evidence for a deviation from the best-fit timing model. We demonstrate that the deviation from the timing model can be modelled as a glitch in the rotation of the pulsar. We exclude instrumental effects and a gravitational wave burst with memory as potential causes, which may induce similar signals in the residuals. We measure the change in spin-frequency due to the glitch to be ∆ν/ν=2.5±0.1×10^(−12) , which is the smallest fractional glitch size reported to date. This is only the second time this phenomenon has been reported in an MSP. We demonstrate that the influence of the glitch can be removed without impacting the timing prospects of the pulsar. We calculate MSP glitch rates and conclude that MSP glitches are rare, and that the probability of another glitch being detected in a timing array pulsar in the next 10 years is ~50%. We measure the DM variations of 31 pulsars that are frequently used as part of pulsar timing arrays, using multi-frequency data sets from DR1 and earlier TOAs recorded with the Lovell Telescope's analogue filterbank. In the best cases, we are able to measure DM variations at a magnitude of ∆DM≳10^(−4) cm^(−3) pc. We compare the values we obtain to those published by the North American Nano-Hertz Observatory for Gravitational Waves (NANOGrav) and the Parkes Pulsar Timing Array (PPTA), and find our measured values to be in generally good agreement. Multi-frequency observations made with the Westerbork Synthesis Radio Telescope (WSRT), were particularly well-suited to obtaining precise DM values, and measurements with this instrument were applied to correct the timing residuals of 10

Date of Award	1 Aug 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Keith (Supervisor) & Benjamin Stappers (Supervisor)