A powerful double radio relic system discovered in PSZ1 G108.18-11.53:
  evidence for a shock with non-uniform Mach number?
  Diffuse radio emission in the form of radio halos and relics has been found
in a number of merging galaxy clusters. These structures indicate that shock
and turbulence associated with the merger accelerate electrons to relativistic
energies. We report the discovery of a radio relic + radio halo system in PSZ1
G108.18-11.53 (z=0.335). This cluster hosts the second most powerful double
radio relic system ever discovered. We observed PSZ1 G108.18-11.53 with the
Giant Meterwave Radio Telescope (GMRT) and the Westerbork Synthesis Radio
Telescope (WSRT). We obtained radio maps at 147, 323, 607 and 1380 MHz. We also
observed the cluster with the Keck telescope, obtaining the spectroscopic
redshift for 42 cluster members. From the injection index we obtained the Mach
number of the shocks generating the two radio relics. For the southern shock we
found M = 2.33^{+0.19}_{-0.26}, while the northern shock Mach number goes from
M = 2.20^{+0.07}_{-0.14} in the north part down to M = 2.00^{+0.03}_{-0.08} in
the southern region. If the relation between the injection index and the Mach
number predicted by diffusive shock acceleration (DSA) theory holds, this is
the first observational evidence for a gradient in the Mach number along a
galaxy cluster merger shock.
