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

Abstract

The cluster 1RXS J0603.3+4214 is a merging galaxy cluster that hosts three radio relics and agiant radio halo. The northern relic, the Toothbrush, is 1.9 Mpc long and has an unusual linearmorphology. According to simple diffusive shock acceleration theory, its radio spectral indexindicates a Mach number of 3.3-4.6. Here, we present results from a deep XMM-Newtonobservation 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 bydensity and temperature discontinuities. However, the observation poses several puzzles thatchallenge our understanding of radio relics: (i) at the Toothbrush, the shock Mach numberis not larger than 2, in apparent conflict with the shock strength predicted from the radiospectrum; (ii) at the Toothbrush, the shock front is, in part, spatially offset from the radioemission; (iii) at the eastern relic, we detect a temperature jump corresponding to a Machnumber of approximately 2.5, but there is no associated surface brightness discontinuity. Wediscuss possible explanations for these findings. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.