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 0.02–0.3 μG. 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.