Investigating the origin of diffuse radio emission in galaxy clusters — using radio and X-ray observations

Galaxy clusters, the largest gravitationally bound objects in the Universe, are assembled through major and minor mergers with other galaxy clusters. Major mergers are the most energetic events since the Big Bang and release vast amounts of energy, as much as 10⁶⁴ ergs. Cluster mergers stir the intracluster medium creating shocks and turbulence, which are illuminated by diffuse radio features called halos, relics, and radio phoenixes. However, shocks and turbulences are detected via signatures in the thermal X-ray emission, e.g., temperature jumps, surface brightness inhomogeneities. Disturbed morphologies in X-ray surface brightness and temperatures are direct evidence for cluster mergers. Therefore, the study of both the X-ray and radio observations can lead to a deeper understanding of the origin of diffuse radio emissions in the galaxy clusters. Here, we present results from multi-wavelength (radio and X-ray) observations of two merging galaxy clusters AS1063 and A85. We produce high fidelity X-Ray temperature maps for both the clusters using the adaptive circular binning (ACB) technique from Chandra X-ray archival data. We report a giant radio halo (using 325 MHz GMRT data) in AS1063. Our detailed analysis has revealed that AS1063 experienced a recent merger, which partially disturbed a weak cool core and formed a moderately disturbed non-cool-core cluster by creating cluster-wide turbulence, which is re-accelerating the relativistic electrons, potentially causing in the formation of the observed radio halo. In A85, low-frequency GMRT observations (325 MHz) have revealed a complex and filamentary diffuse radio phoenix at the place of the infalling southwest subcluster. We also detect an X-ray shock near to the radio phoenix. For the first time, we find the observational evidence of the possible connection between an X-ray shock and the radio phoenix in A85.