Investigating pulsar emission variability using the MeerKAT radio telescope

Abstract

Since the discovery of pulsars more than 50 years ago, pulsars have revealed complicated radio emission phenomena on short timescales of a few single pulses, on timescales comparable to the length of observations, or even longer. As pulsars are weak radio sources, pulsar studies have been aided by ever larger radio telescopes, such as provided by interferometers with numerous single dishes. Effectively scheduling pulsar observations has become increasingly important with ever larger telescopes, and their ability to form subarrays provides further opportunities. Therefore, we develop a simple scheme to find an optimal observing time for a given pulsar. The designed scheme guarantees a long enough observing time to obtain a high fidelity pulse profile. This ensures sufficient precision to, for example, characterise pulse shape variations in different observations and to perform pulsar timing experiments. We test the scheme with data obtained from the MeerKAT radio telescope. The method is used for the Thousand-Pulsar-Array (TPA) long-term timing project, for which we find that using two 32-dish MeerKAT subarrays is the most efficient observing strategy to monitor a large number of pulsars. However, different subarray configurations can be beneficial for other observing programmes. In addition to the pulsar monitoring campaign, the TPA project obtained one-off high quality and signal-to-noise ratio (S/N) data for over 1000 pulsars. These data are particularly suitable for single pulse studies. We adopt a systematic approach with a semi-automated pipeline to study the subpulse modulation properties of 1198 pulsars, representing a large fraction of the known pulsar population. We find that many pulsars show periodic pulse shape variability in the form of drifting subpulses, and demonstrate that this phenomenon is common such that it is detectable in 60% of all pulsars if sufficient S/N is available. This unprecedented large systematic study allows the evolution of drifting subpulses to be studied over the lifetime of pulsars. In particular it is found that the modulation period P_3 is evolving in a non-monotonic way. This can be naturally explained if aliasing occurs for young pulsars, who have intrinsically the fastest modulation. Although this explanation is opposite to the prediction of the standard drifting subpulse model, it can be reconciled if the polar cap physics is driving a beat frequency. Pulsar timing has revealed rotational instabilities in most pulsars. The spin-down rate often reveals a quasi-periodic behaviour and the integrated pulse profile of some pulsars is seen to switch between different stable forms. Remarkably, the changes in the spin-down rate and pulse shape are correlated for a small number of pulsars, and therefore are both thought to be driven by the same magnetospheric process. One of these pulsars, PSR J0742-2822, is studied in detail. An enhanced correlation between the spin-down rate evolution and pulse profile variations is seen after a large glitch. Our analysis uses an extended dataset, and Principal Component Analysis to quantify profile variability. The correlation is found to become irregular after another glitch, making it unclear if and how the spin-down rate evolution, pulse shape variations and glitches are associated. We show that the relation between spin down and profile variability is more complicated than previously reported, and cannot be explained by a two-state magnetospheric switching model.

Date of Award	23 Feb 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Patrick Weltevrede (Main Supervisor) & Michael Keith (Co Supervisor)