Lucy Oswald

Abstract:

ABSTRACT The frequency widening of pulsar profiles is commonly attributed to lower frequencies being produced at greater heights above the surface of the pulsar; so-called radius-to-frequency mapping (RFM). The observer’s view of pulsar emission is a 1D cut through a 3D magnetosphere: we can only see that emission which points along our line of sight. However, by comparing the frequency evolution of many single pulses positioned at different phases, we can build up an understanding of the shape of the active emission region. We use single pulses observed with the Giant Metrewave Radio Telescope to investigate the emission region of PSR J1136+1551 and test RFM. Assuming that emission is produced tangential to the magnetic field lines and that each emission frequency corresponds to a single height, we simulate the single pulse profile evolution resulting from the canonical conal beam model and a fan beam model. Comparing the results of these simulations with the observations, we conclude that the emission region of PSR J1136+1551 is better described by the fan beam model. The diversity of profile widening behaviour observed for the single pulses can be explained by orthogonally polarized modes propagating along differing frequency-dependent paths in the magnetosphere.

Abstract:

We report modelling follow-up of recently discovered gravitational-lens candidates in the Canada France Hawaii Telescope Legacy Survey. Lens modelling was done by a small group of specially interested volunteers from the SpaceWarps citizen-science community who originally found the candidate lenses. Models are categorized according to seven diagnostics indicating (a) the image morphology and how clear or indistinct it is, (b) whether the mass map and synthetic lensed image appear to be plausible, and (c) how the lens-model mass compares with the stellar mass and the abundance-matched halo mass. The lensing masses range from ~10 11 to > 10 13 M ⊙ . Preliminary estimates of the stellar masses show a smaller spread in stellar mass (except for two lenses): a factor of a few below or above ~10 11 M ⊙ . Therefore, we expect the stellar-to-total mass fraction to decline sharply as lensing mass increases. The most massive system with a convincing model is J1434+522 (SW 05). The two low-mass outliers are J0206-095 (SW 19) and J2217+015 (SW 42); if these two are indeed lenses, they probe an interesting regime of very low star formation efficiency. Some improvements to the modelling software (SpaghettiLens), and discussion of strategies regarding scaling to future surveys with more and frequent discoveries, are included.

Abstract:

The canonical model of a pulsar is insufficient to describe the variety and variability of its radio emission. Pulsars are neutron stars, with intense gravitational and magnetic fields, which emit bright beams of radio waves that co-rotate with the spinning star. The regularity of pulsar rotation, and hence the arrival times of pulses of radio emission, means that pulsars are used as clocks in space to test fundamental theories of physics and to search for gravitational waves. However, their accuracy as clocks is limited by the fact that we do not fully understand how pulsars produce their radio beams, and so cannot predict their emission behaviour completely.

Throughout the history of pulsar science, new telescopes and updated technology have expanded the complexity observable in the shapes and properties of observed pulse profiles from the radio pulsar population. This growing dataset has raised as many new questions as it has answered about the nature of pulsar radio emission, the properties of the pulsar population and our ability to characterize the behaviour of pulsars and their environments with physical laws. However, modern instruments offer a broad-band view of radio pulsars. This allows us to probe, for the first time, the continuous evolution of pulsar radio emission over a wide frequency range with a single instrument.

This thesis uses the expanded observational capabilities of new broad-band instruments to make progress in answering fundamental questions about pulsar radio emission. I apply a statistical approach to the frequency evolution of single pulses of PSR J1136+1551 observed by the GMRT, in order to constrain the cross-section and frequency-dependent emission heights of its radio beam structure. This work shows that the beam structure of J1136+1551 is best described by a fan beam model and that it is important to include the effects of orthogonal polarization mode interaction to explain the frequency-dependent behaviour. In order to maximize the understanding available with broad-band observations, it is important to address the interaction between pulsar radio emission and the interstellar medium through which it propagates. I create an algorithm for correcting the effects of the interstellar medium in broad-band polarimetric data from the Parkes Ultra-Wideband receiver to reveal the intrinsic polarization behaviour of PSRs~J1056--6258 and J1359--6038. Finally, I apply understanding of intrinsic pulsar behaviour to constrain models of pulsar scattering by the interstellar medium, performing a survey of the scattering properties of 84 single-component pulsars observed with the MeerKAT telescope.

This work reveals the capacity of new broad-band observations to expand our understanding of pulsar radio emission. It constrains understanding of both pulsar beam structure, including frequency-dependent emission heights, and the structures that make up the interstellar medium. The results highlight the importance of accounting for the behaviour of both the pulsar and the interstellar medium simultaneously when analysing broad-band observations, and future work will focus on applying these modelling approaches to pulsars with complex profile shapes and polarization properties.