Proton Acceleration in Intracluster Shocks and Gamma ray and Neutrino Emissions from Galaxy Clusters

Abstract

Shock waves with low sonic Mach number (Ms) are induced by mergers and/or flow motions in the hot tenuous plasmas of the intracluster medium (ICM). High-energy cosmic ray (CR) protons are expected to be accelerated at quasi-parallel shocks via diffusive shock acceleration (DSA), while proton acceleration is suppressed at quasi-perpendicular shocks. A key element of DSA is the so-called injection process, which energizes thermal protons to the suprathermal energies sufficient to diffuse across the shock. We first present a study of CR injection and early acceleration at weak quasi-parallel shocks using particle-in-cell (PIC) simulations. We then propose an analytic model for the quantitative measures of CR proton acceleration in ICM shocks in the test-particle regime. The model suggests that the acceleration efficiency of CR protons ranges ~ 0.001 - 0.02 in supercritical shocks with Ms ~ 2.25 - 5, and the acceleration would be negligible in subcritical shocks with Ms < ~2.25. We then estimate the emissions of gamma-rays and neutrinos from galaxy clusters. The predicted gamma-ray flux is below the upper limit set by Fermi-LAT. The neutrino flux towards clusters like the Coma cluster would be about ~ 1% of that of atmospheric neutrinos at ~ 100 GeV. We discuss the implication of our results.