Nonthermal Origin of the EUV and Soft X-Rays from the Coma Cluster:Cosmic Rays in Equipartition with the Thermal Medium

Abstract

The role of cosmic rays (CRs) in the formation and evolution of clusters of galaxies has been much debated. It may well be related to other fundamental questions, such as the mechanism that heats and virializes the intracluster medium (ICM) and the frequency at which the ICM is shocked. There is now compelling evidence, both from the cluster soft excess (CSE) and the hard-tail emissions at energies above 10 keV, that many clusters are luminous sources of inverse Compton (IC) emission. This is the first direct measurement of cluster CRs: the technique is free from our uncertainties in the ICM magnetic field and is not limited to the small subset of clusters that exhibit radio halos. The CSE-emitting electrons fall within a crucial decade of energy where they have the least spectral evolution and where most of the CR pressure resides. However, their survival times do not date them back to the relic CR population. By using the CSE data of the Coma Cluster, we demonstrate that the CRs are energetically as important as the thermal ICM: the two components are in pressure equiparition. Thus, contrary to previous expectations, CRs are a dominant component of the ICM, and their origin and effects should be explored. The best-fit CR spectral index is in agreement with the Galactic value.