The origin of the diffuse non-thermal X-ray and radio emission in the Ophiuchus cluster of galaxies

Abstract

We present high-resolution 240 and 607 MHz Giant Metrewave Radio Telescope radio observations, complemented with 74 MHz archival Very Large Array radio observations of the Ophiuchus cluster of galaxies, whose radio mini-halo has been recently detected at 1400 MHz. We also present archival Chandra and XMM-Newton data of the Ophiuchus cluster. Our observations do not show significant radio emission from the mini-halo, hence we present upper limits to the integrated, diffuse non-thermal radio emission of the core of the Ophiuchus cluster. The XMM-Newton observations can be well explained by a two-temperature thermal model with temperatures of ~=1.8 and ~=9.0 keV, respectively, which confirms previous results that suggest that the innermost central region of the Ophiuchus cluster is a cooling core. This result is consistent with the occurrence of a mini-halo, as expected to be found in hot clusters with cool cores. We also used the XMM-Newton data to set up an upper limit to the (non-thermal) X-ray emission from the cluster. We also emphasize that the non-thermal X-ray emission obtained with XMM-Newton and International Gamma-Ray Astrophysics Laboratory (INTEGRAL) cannot be produced by the putative active galactic nucleus of the galaxy at the cluster centre. The combination of available radio and X-ray data has strong implications for the currently proposed models of the spectral energy distribution (SED) from the Ophiuchus cluster. In particular, a synchrotron+inverse Compton model is in agreement with the currently available data, if the average magnetic field is in the range of 0.02-0.3μG. A pure weakly interacting massive particle annihilation scenario can in principle reproduce both radio and X-ray emission, but at the expense of postulating very large boost factors from dark matter substructures, jointly with extremely low values of the average magnetic field. Finally, a scenario where synchrotron and inverse Compton emission arise from PeV electron-positron pairs (via interactions with the cosmic microwave background) can be ruled out, as it predicts a non-thermal soft X-ray emission that largely exceeds the thermal bremsstrahlung measured by INTEGRAL.