Cosmic Rays in Galaxy Clusters

Abstract

Like in our galaxy, cosmic rays (CRs) exist in clusters of galaxies. Shocks waves induced during the hierarchical formation of the large-scale structure (LSS) of the universe are believed to be the dominant sources of those intergalactic CRs. The presence of such shocks have been indicated by X-ray and radio observations of merging clusters. In particular, diffuse radio sources known as radio relics could be explained by synchrotron-emitting electrons produced at cluster shocks via the diffusive shock acceleration (DSA). Yet, there remain unresolved puzzles, such as the discrepancy in the shock Mach numbers inferred from the radio spectral index and estimated from X-ray observations for some radio relics, and the lack of observation of pion0 decay gamma-ray from inelastic collisions of shock-accelerated protons with thermal protons in clusters. We here discuss the nature of cluster shocks using hydrodynamics/N-body simulations for the LSS formation and the acceleration of CRs at the shocks with PIC (particle-in-cell) simulations.