Ultrahigh energy cosmic rays and the GeV-TeV diffuse gamma-ray flux

Ultra-high energy cosmic rays accelerated in astrophysical objects produce secondary electromagnetic cascades during propagation in the cosmic microwave and infrared backgrounds. We show that if the primary cosmic rays are dominated by protons, such cascades can contribute between ≃1% and ≃50% of the GeV-TeV diffuse photon flux measured by the EGRET experiment. The Fermi Gamma-ray Space Telescope (GLAST) should have a good chance to discover this flux. If most ultra-high energy cosmic rays consist of heavy nuclei, the secondary GeV-TeV diffuse photon flux can be lower by factors of several compared to pure proton primaries from the same sources. The diffuse photon flux then depends on the unknown composition of accelerated particles, their maximum energy, and the source distribution.