Hard X-Ray and Gamma-Ray Emission Induced by Ultra-High-Energy Protons in Cluster Accretion Shocks

All sufficiently massive clusters of galaxies are expected to be surrounded by strong accretion shocks, where protons can be accelerated to ~10¹⁸-10¹⁹ eV under plausible conditions. Such protons interact with the cosmic microwave background and efficiently produce very high energy electron-positron pairs, which then radiate synchrotron and inverse Compton emission, peaking respectively at hard X-ray and TeV gamma-ray energies. Characterized by hard spectra (photon indices ~1.5) and spatial distributions tracing the accretion shock, these can dominate over other nonthermal components depending on the shock magnetic field. HESS and other Cerenkov telescopes may detect the TeV emission from nearby clusters, notwithstanding its extended nature. The hard X-rays may be observable by future imaging facilities such as NeXT and possibly also by the Astro-E2 Hard X-Ray Detector. Such detections will provide not only a clear signature of ultra-high-energy proton acceleration, but also an important probe of the accretion shock itself, as well as magnetic fields in the outermost regions of clusters.