Shock heating of the merging galaxy cluster A521

TL;DR

This study uses XMM-Newton observations to identify shock fronts in galaxy cluster A521, linking shock properties to radio relic features and cosmic ray acceleration mechanisms.

Contribution

It provides the first detailed analysis of shock fronts in A521, confirming their role in radio relic formation and cosmic ray acceleration.

Findings

Detection of two shock fronts with Mach number ~2.4

Spatial correlation between shock fronts and radio relics

Identification of cold fronts separating cluster cores

Abstract

A521 is an interacting galaxy cluster located at z=0.247, hosting a low frequency radio halo connected to an eastern radio relic. Previous Chandra observations hinted at the presence of an X-ray brightness edge at the position of the relic, which may be a shock front. We analyze a deep observation of A521 recently performed with XMM-Newton in order to probe the cluster structure up to the outermost regions covered by the radio emission. The cluster atmosphere exhibits various brightness and temperature anisotropies. In particular, two cluster cores appear to be separated by two cold fronts. We find two shock fronts, one that was suggested by Chandra and that is propagating to the east, and another to the southwestern cluster outskirt. The two main interacting clusters appear to be separated by a shock heated region, which exhibits a spatial correlation with the radio halo. The outer edge of the radio relic coincides spatially with a shock front, suggesting this shock is responsible for the generation of cosmic ray electrons in the relic. The propagation direction and Mach number of the shock front derived from the gas density jump, M = 2.4 +/- 0.2, are consistent with expectations from the radio spectral index, under the assumption of Fermi I acceleration mechanism.