COSMIC RAY PROTON ACCELERATION AT WEAK ICM SHOCKS AND OBSERVATIONAL SIGNATURES

Abstract

Weak shocks with low sonic Mach numbers are induced by mergers and/or supersonic flow motions in the hot tenuous intracluster medium (ICM). High energy cosmic ray (CR) protons are expected to be accelerated at quasi-parallel shocks via diffusive shock acceleration (DSA). The key element in determining the DSA efficiency is the so-called injection process, which energizes thermal particles to suprathermal energies sufficient to diffuse across the shock transition layer. Reflection of incoming particles and ensuing self-excitation of MHD/plasma waves play important roles in the particle injection to DSA. Using particle-in-cell (PIC) simulations and analytic modeling, we study the injection and acceleration of protons at weak shocks in high-beta plasma. We find that the proton injection at quasi-parallel shocks is effective only for supercritical shocks with the sonic Mach number greater than about 2.25, and the acceleration efficiency at such shocks ranges from 0.001 to 0.01. We then quantify the CR protons produced at ICM shocks in simulated galaxy clusters during the large-scale structure formation. Finally, we estimate diffuse gamma-ray and neutrino emissions, resulting from inelastic collisions between CR protons and thermal protons, and evaluate the prospect of their observations.