On the Polarisation of Radio Relics

TL;DR

This paper investigates the origin of radio relic polarisation in galaxy clusters, proposing a toy model that links magnetic field strength and shock properties to observed polarisation features, supporting the shock compression scenario.

Contribution

The study introduces a toy model for radio relic polarisation based on shock compression, highlighting the role of magnetic field strength and challenging the significance of shock strength.

Findings

Magnetic field strength crucially affects fractional polarisation.

Shock strength has little impact on overall polarisation fraction.

Polarisation may decrease downstream depending on magnetic field strength.

Abstract

Radio relics are extended radio emission features which trace shock waves in the periphery of galaxy clusters originating from cluster mergers. Some radio relics show a highly polarised emission, which make relics an excellent probe for the magnetisation of the intra-cluster medium. The origin of the relic polarisation is still debated. It could be a result of tangentially stretching the magnetic field at the shock surface. This scenario would naturally explain the alignment of the polarisation (E-vectors) with the shock normal. We have implemented a toy model for the relic polarisation according to this scenario. We find that the magnetic field strength itself crucially affects the fractional polarisation. Moreover, we find that the shock strength has surprisingly little effect on the overall polarisation fraction. Finally, we find that the fractional polarisation may decrease downstream depending on the magnetic field strength. Our results demonstrates that the shock compression scenario provides a very plausible explanation for the radio relic polarisation which specific features permitting to test the origin of radio relic polarisation.