Nonthermal Emissions from the Outer Region of Galaxy Clusters

Abstract

The cosmic web is known to emit nonthermal radiation through various processes associated with shocks, cosmic rays (CRs), and magnetic fields. In particular, galaxy clusters are surrounded by strong accretion shocks induced by gas infall from the filaments and voids. These shocks are characterized by high Mach numbers and propagate into a weakly magnetized medium. Although they are expected to be efficient CR accelerators, the detailed acceleration processes at such high-beta high-Mach-number shocks have yet to be fully understood. In this study, we used particle-in-cell kinetic simulations to show that strong, high-beta shocks could be mediated via the ion Weibel instability, and that electrons can be pre-accelerated via the stochastic Fermi process to be injected into diffusive shock acceleration. Based on these findings, we propose analytic models for the energy spectra of shock-accelerated CR protons and electrons that utilize conventional thermal leakage injection concept. Applying these model CR spectra to numerical shock zones in structure formation simulations, we estimated nonthermal emissions, including synchrotron and inverse Compton radiation due to CR electrons and pi^0-decay gamma-rays due to CR protons, around simulated clusters. The synthetic synchrotron maps produced by the models are consistent with recent radio observations of the Coma Cluster. However, detecting nonthermal inverse Compton X-rays and gamma-rays from accretion shocks in the outer region of galaxy clusters with currently available observational facilities would be challenging.