Radio Polarization Observations of the Snail: A Crushed Pulsar Wind Nebula in G327.1-1.1 with a Highly Ordered Magnetic Field

TL;DR

This study presents radio polarization observations of the Snail PWN, revealing a highly ordered magnetic field contrary to expectations, and suggests large turbulence scales and significant mixing with supernova ejecta.

Contribution

It provides the first detailed polarization analysis of an evolved PWN post-reverse shock, highlighting magnetic field order and turbulence scale implications.

Findings

Detected strong linear polarization indicating ordered magnetic fields

Model suggests turbulence scale is about one-eighth to one-sixth of nebula radius

Implied significant mixing between supernova ejecta and pulsar wind

Abstract

Pulsar wind nebulae (PWNe) are suggested to be acceleration sites of cosmic rays in the Galaxy. While the magnetic field plays an important role in the acceleration process, previous observations of magnetic field configurations of PWNe are rare, particularly for evolved systems. We present a radio polarization study of the "Snail" PWN inside the supernova remnant G327.1-1.1 using the Australia Telescope Compact Array. This PWN is believed to have been recently crushed by the supernova (SN) reverse shock. The radio morphology is composed of a main circular body with a finger-like protrusion. We detected a strong linear polarization signal from the emission, which reflects a highly ordered magnetic field in the PWN and is in contrast to the turbulent environment with a tangled magnetic field generally expected from hydrodynamical simulations. This could suggest that the characteristic turbulence scale is larger than the radio beam size. We built a toy model to explore this possibility, and found that a simulated PWN with a turbulence scale of about one-eighth to one-sixth of the nebula radius and a pulsar wind filling factor of 50--75% provides the best match to observations. This implies substantial mixing between the SN ejecta and pulsar wind material in this system.