Non-thermal Origin of the EUV and HEX Excess Emission of the Coma Cluster - the Nature of the Energetic Electrons

Abstract

The excess in extreme-ultraviolet (EUV) radiation and high energy X-rays (HEX) of Coma may be modeled by power law spectra. Their origin could be inverse Compton (IC) emission by relativistic electrons. The scattered photon fields can either be the CMB or the starlight of the galaxies within the cluster. If the EUV excess is due to CMB-IC, a strongly inhomogeneous magnetized intra- cluster medium (ICM) is required, in which the density of the relativistic electrons is anticorrelated with the magnetic fields. This could be understood if the electrons were accelerated during a major merger event within the last 2 Gyr and cooled afterwards in the inhomogeneous fields. If the EUV excess is due to starlight-IC, a population of relativistic, very low energy electrons has to be present, with a high energy density. In order to survive Coulomb losses, these electrons have to be separated from the cluster gas by confining magnetic fields. This could be remnant radio plasma from the epoch of violent quasar activity. The observed narrow radial profile of the EUV excess emission is due to the narrow photon distribution for this model. Both models favor very inhomogeneous magnetic field and relativistic electron distributions. IC models for the HEX excess seem to be implausible: CMB-IC leads to a mainly unmagnetized ICM, in contradiction to Faraday rotation measurements. Starlight scattering electrons would overproduce EUV photons due to simultaneously CMB-IC. We propose that the observed HEX excess is due to bremsstrahlung of a small high energy power-law tail of the mainly thermal ICM electron distribution. Such a tail is expected due to in-situ acceleration by turbulence.