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