2023MNRAS.522.2756L

With tantalizing evidence of the recent e-Rosita mission, re-discovering very soft X-rays and Extreme-Ultraviolet (EUV) radiation from a cluster of galaxies or its environment, the question of the origin of cluster EUV excess is revisited in this work. It will be shown that the gas temperature, density, and frozen-in magnetic field of the intracluster medium, collectively support the emission and propagation of coherent C̆erenkov radiation, which is low frequency and large amplitude radiation capable of accelerating charged particles to relativistic speeds. Owing to the spectrum of C̆erenkov radiation, most of the incipient relativistic electrons undergo inverse-Compton scattering with the cosmic microwave background. It turns out the scattered radiation has observable ramifications only in the EUV band, of photon energy 70-100 eV, having a luminosity ≈10⁴⁴ ergs s^–1. This luminosity is on par with the EUV excess level detected from Abell 1795 and the Coma cluster. It should be stressed, as caveat emptor, that although the main subject is the putative large amplitude coherent C̆erenkov modes which are highly non-linear, the results presented were derived using a quasi-linear approach to highlight the observable features of the phenomenon, namely, the EUV emission.