Particle Acceleration at weak shocks in high beta ICM plasma

Abstract

Weak shocks with low sonic Mach number 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), while CR electrons are expected to be accelerated preferentially at quasi-perpendicular shocks. The key element in determining the DSA efficiency is the so-called injection process, which energizes thermal particle 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, we study CR injection and early acceleration at weak shocks in high beta plasma. We find that the proton (electron) injection at quasi-parallel (quasi-perpendicular) shocks is effective only for supercritical shocks with the sonic Mach number greater than about 2.3. Further investigation is