Diffuse radio emission in MACS J1752.0+4440
  We report the discovery of large-scale diffuse radio emission in the galaxy
cluster MACS J1752.0+4440 (z=0.366). Using Westerbork Synthesis Radio Telescope
(WSRT) observations we find that the cluster hosts a double radio relic system
as well as a 1.65 Mpc radio halo covering the region between the two relics.
The relics are diametrically located on opposite sides of the cluster center.
The NE and SW relics have sizes of 1.3 and 0.9 Mpc, respectively. In case of an
isolated binary merger event, the relative sizes of the relics suggest a mass
ratio about 2:1. We measure integrated spectra of -1.16 \pm 0.03 for the NE and
-1.10 \pm 0.05 for the SW relic. We conclude that this cluster has undergone a
violent binary merger event and the relics are best explained by particles
(re)accelerated in outwards traveling shock waves. The spectral indices suggest
the relics trace shock waves with Mach numbers (M) of around 3.5 to 4.5. These
relatively high Mach numbers derived from the radio spectral index are
comparable to those derived for a few other recently discovered relics. This
implies that merger shocks with M > 3 are relatively common in cluster
outskirts if our understanding of diffusive shock acceleration is correct.
