The coma cluster at γ-ray energies: Multifrequency constraints

Abstract

The Coma cluster exhibits evidence of a high-energy non-thermal particle population. At frequencies > 1 GHz recent radio halo observations confirm a significant spectral steepening of the volume-integrated emission. We calculate the volume-averaged high-energy spectrum due to inverse Compton scattering off the CMB radiation field and non-thermal bremsstrahlung according to an exponential cutoff in the electron spectrum as deduced from the radio observations. The synchrotron radiation from secondary pairs, created from the decay of charged mesons produced in hadronic pp-interactions, is found to set significant constraints on the energy content of relativistic hadrons in Coma. This limits the maximum flux at high energies. Our findings support a low ratio of relativistic hadron to thermal energy density. Predictions for Coma's high energy emission are discussed in the light of current and expected abilities of upcoming γ-ray instruments.