Cosmic gamma rays and neutrinos of energy near 1020 eV

In recent papers the authors have followed up the common hypothesis that cosmic ray particles above about 1019 eV are mainly of extragalactic origin by evaluating the consequences for the associated cosmic gamma rays and neutrinos. Here they derive the expected fluxes of gamma rays and neutrinos, with energy of order 1020 eV, arising from the interaction of the protons with microwave background photons. The derived ratio of gamma rays to protons at 1020 eV is approximately 0.03, increasing to 0.15 at 5*1020 eV, for the situation where the cosmic ray sources are distributed nearly uniformly throughout the Universe; the corresponding figures for the neutrino to proton are factors of 5 and 24, respectively. An alternative, or perhaps additive, view is that most of the protons detected come via slow diffusion from the local Virgo cluster of galaxies. Here, significant fluxes of very energetic gamma rays and neutrinos are expected from the direction of this cluster. Specifically, for one steradian central on Virgo, the predicted gamma to proton ratio is approximately 0.2 at 1020 eV increasing to approximately 1.3 at 5*1020 eV. For the same energies, the neutrino to proton ratios are approximately=12 and 75.