Shock Acceleration Model for Radio Relics in Galaxy Clusters

Abstract

Radio relics are diffuse radio sources found in the outskirts of galaxy clusters. Although most of their properties could be explained by synchrotron-emitting electrons accelerated via diffusive shock acceleration (DSA) at merger-driven shocks, there remain a few puzzles. For some radio relics, the shock Mach number inferred from the radio spectral index is larger than that estimated from the discontinuities in X-ray observations. Such discrepancy could be understood, if the shock Mach number is under-estimated in X-ray observation due to projection effects, or if pre-existing electrons with a flat energy spectrum is re-accelerated by a weak shock. Here we explore these two scenarios by comparing the results of DSA simulations with observed features of a few radio relics. Toward this end, we perform DSA simulations of spherical shocks with the parameters relevant for cluster radio relics and calculate the ensuing radio synchrotron emission from accelerated electrons.