Nonthermal Particles and Radiation Produced by Cluster Merger Shocks

We have developed a numerical model for the temporal evolution of particle and photon spectra resulting from nonthermal processes at the shock fronts formed in merging clusters of galaxies. Fermi acceleration is approximated by injecting power-law distributions of particles during a merger event, subject to constraints on maximum particle energies. We consider synchrotron, bremsstrahlung, Compton, and Coulomb processes for the electrons, nuclear, photomeson, and Coulomb processes for the protons, and knock-on electron production. Broadband radio through gamma-ray light curves radiated by nonthermal protons and primary and secondary electrons are calculated both during and after the merger event. Using ROSAT observations to establish typical parameters for the matter density profile of clusters of galaxies, we find that merger shocks are weak and accelerate particles with relatively soft spectra. Our results suggest that only a minor contribution to the diffuse extragalactic gamma-ray background can originate from cluster merger shocks.