Radio polarisation study of the snail pulsar wind nebula in supernova remnant G327.1-1.1

Abstract

Pulsar wind nebulae (PWNe) are believed to be acceleration sites of cosmic rays in the Galaxy. In the acceleration process, magnetic field plays an important role. Radio polarisation measurements offer a direct probe of the magnetic field geometry of astronomical objects, but such experiments have rarely been conducted on PWNe, especially for evolved systems. PWNe can be crushed by the supernova reverse shock at an age of ~ 10 kyr. Previous hydrodynamical simulations show that such interactions can result in a turbulent environment in the nebula interior, suggesting a tangled magnetic field.

In this thesis, I present a radio study of the Snail PWN in the composite supernova remnant G327.1−1.1 using the Australia Telescope Compact Array. This PWN is believed to have already interacted with the supernova reverse shock. The study reveals a highly ordered magnetic field configuration in the Snail, which can be explained if either the shockwave could not penetrate into the PWN interior to drive the turbulence, or the characteristic turbulence scale is large. A toy model is built to estimate the turbulence scale assuming the latter scenario. It is found that a simulated PWN with a turbulence scale of one-eighth to one-sixth of the nebula radius and a pulsar wind filling factor of 50–75% can match the observation results. This suggests significant mixing between supernova ejecta and pulsar wind material in this system.

In addition, the Snail exhibits a subsonic comet-like protrusion extending from the putative neutron star. Cometary PWNe were found to exhibit a variety of magnetic field configurations but the exact reason remains unclear. The polarisation observations of this system revealed a magnetic field parallel to the nebula elongation, similar to what was found in the Mouse (G359.23−0.82) and the handle of the Frying Pan (G315.78−0.23). This adds an important sample to cometary PWNe for future MHD modelling.