2012MNRAS.425L..36V

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 centre. The NE and SW relics have sizes of 1.3 and 0.9 Mpc, respectively. In the 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 ±0.03 for the NE relic and -1.10±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 travelling shock waves. The spectral indices suggest that the relics trace shock waves with Mach numbers (M) of around 3.5–4.5. 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.