Diffuse Flux of Ultra-high-energy Photons from Cosmic-Ray Interactions in the Disk of the Galaxy and Implications for the Search for Decaying Super-heavy Dark Matter

Abstract

Abstract An estimate of the expected photon flux above 1017 eV from the interactions of ultra-high-energy cosmic rays with the matter in the Galactic disk is presented. Uncertainties arising from the distribution of the gas in the disk, the absolute level of the cosmic-ray flux, and the composition of the cosmic rays are taken into account. Within these uncertainties, the integrated photon flux above 1017 eV is averaged out over Galactic latitude less than 5°, between ≃3.2 × 10−2 km−2 yr−1 sr−1 and ≃8.7 × 10−2 km−2 yr−1 sr−1. The all-sky average value amounts to ≃1.1 ×10−2 km−2 yr−1 sr−1 above 1017 eV and decreases roughly as E −2, making this diffuse flux the dominant one from cosmic-ray interactions for energy thresholds between 1017 and 1018 eV. Compared to the current sensitivities of detection techniques, a gain of between two and three orders of magnitude in exposure is required for a detection below ≃1018 eV. The implications for searches for photon fluxes from the Galactic center that would be indicative of the decay of super-heavy dark matter particles are discussed, as the photon flux presented in this study can be considered as a floor below which other signals would be overwhelmed.