Challenges to our understanding of radio relics: X-ray observations of the Toothbrush cluster

TL;DR

This study uses deep X-ray observations to investigate the complex shock structures and properties of radio relics in the merging galaxy cluster 1RXS J0603.3+4214, revealing discrepancies with existing theories of radio relic formation.

Contribution

It provides new X-ray evidence of shock properties and challenges current models of radio relics, especially regarding shock Mach numbers and their relation to radio emission.

Findings

Shock Mach number at the Toothbrush is lower than predicted by radio spectra.

Spatial offset observed between shock front and radio emission.

Temperature jump detected without corresponding surface brightness discontinuity.

Abstract

The cluster 1RXS J0603.3+4214 is a merging galaxy cluster that hosts three radio relics and a giant radio halo. The northern relic, the Toothbrush, is 1.9-Mpc long and has an unusual linear morphology. According to simple diffusive shock acceleration theory, its radio spectral index indicates a Mach number of 3.3-4.6. Here, we present results from a deep XMM-Newton observation of the cluster. We observe two distinct cluster cores that have survived the merger. The presence of three shocks at or near the locations of the radio relics is confirmed by density and temperature discontinuities. However, the observation poses several puzzles that challenge our understanding of radio relics: (i) at the Toothbrush, the shock Mach number is not larger than 2, in apparent conflict with the shock strength predicted from the radio spectrum; (ii) at the Toothbrush, the shock front is, in part, spatially offset from the radio emission; (iii) at the eastern relic, we detect a temperature jump corresponding to a Mach number of approximately 2.5, but there is no associated surface brightness discontinuity. We discuss possible explanations for these findings.