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Pulsar Polarisation As A Diagnostic Tool
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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Abstract
The geometry of pulsar beams is one of the intrinsic properties of neutron stars, governing the pulse-profile phenomenon and other aspects of pulsar astron- omy. With a number of pulsars in our dataset, their beam geometry is derived from the polarisation position angle (PPA) using the simple polar cap emission and dipole field model. This includes the rotating vector model (RVM), for which the solutions can hardly be constrained or fail to be consistent because of the lim- itations of the model itself. The inconsistencies in the results suggest that the initial PPAs can be strongly perturbed by additional parameters above the emis- sion altitude, such as the plasma medium or rotational aberration effects, after which their characteristic shape is no longer related to the geometry via the RVM. We investigate further into the effects of wave propagation in the pulsar magne- tosphere, and find an indication that, in most cases, the RVM-calculated PPAs are likely to be altered by plasma effects.In recent years, there have been an increasing number of intermittent and mode-switching pulsars observed to have their radio pulse profiles correlated with the change in pulsar spin frequency (ν ̇) (e.g. Lorimer et al. 2012, Lyne et al. 2010). These two phenomena are understood to be related via the states of plasma in the magnetosphere. As one such pulsar, and also one with known geometry and other astonishing behaviour, PSR B1822–09 is studied in terms of the mode- switching properties, the hollow-cone model and the wave propagation in the magnetosphere. We also study the model for explaining the intermittent pulsars PSRs B1931+24, J1841+0500 and J1832+0029, and find it can be consistently applied for PSRs B1822–09 and B0943+10, and other profile-switching pulsars. However, aspects of the conclusions are limited because of the lack of understand- ing of the connection between the radio flux and the states of plasma. We are also able to use the difference in the PPAs between two states of PSR B0943+10 to predict the change in plasma states and ν ̇, which cannot be measured directly from timing analysis as its switching timescale is too short.