Hard X-ray and Gamma-ray Emission Induced by Ultra-High Energy Protons   in Cluster Accretion Shocks and their Detectability
  Protons can be plausibly accelerated to $\sim 10^{18}$-$10^{19}$ eV in strong accretion shocks surrounding massive clusters of galaxies. Such protons efficiently produce very high energy pairs through interactions with the CMB, which then radiate synchrotron and inverse Compton emission with hard spectra, peaking respectively in hard X-rays and TeV gamma-rays. We discuss the prospects for detecting this emission from nearby clusters with current and future facilities such as HESS and NeXT, which should provide a clear signature of ultra-high energy proton acceleration and an important probe of magnetic fields in the outermost regions of clusters. 
Susumu Inoue, Felix A. Aharonian, Naoshi Sugiyama