The Mpc-scale diffuse radio emission that is observed in some of the merging galaxy clusters, both in the form of radio halos and relics, carries crucial information about the presence of non-thermal components in the intracluster medium, such as cosmic rays and magnetic fields, and about particle acceleration processes by shocks and turbulence. ACO2163, which is one of the clusters which show merging activity, is also one of the hottest and richest of the Abell galaxy clusters. The cluster has been observed countless times through many different wavelengths and has always shown remarkable properties.
In this paper we report on a discovery of a shock front in ACO2163, detected in the XMM-Newton data, with a Mach number of 2.2±0.3 obtained from temperature jump measurements. This Mach number is in good agreement with that implicated by the injection spectral index at the shock front from the radio observations, in combination with Diffusive Shock Acceleration.
We have also used the Kang & Ryu model to explain the electrons at the shock front, and the observed discrepancy between the average spectral index of the halo and that predicted by the M = 2.2 shock. In the scenario proposed by this model, the shock in ACO2163 is passing through a cloud of fossil relativistic electrons which were provided by a nearby radio galaxy, re-accelerating the fossil relativistic electrons in the fossil cloud, and will exit out of the cloud at cloud crossing time of ∼54 Myr.