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

An estimate of the expected photon flux above 10(17) 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 10(17) eV is averaged out over Galactic latitude less than 5 degrees, between similar or equal to 3.2 x 10(-2) km(-2) yr(-1) sr(-1) and similar or equal to 8.7 x 10(-2) km(-2) yr(-1) sr(-1). The all-sky average value amounts to similar or equal to 1.1 x10(-2) km(-2) yr(-1) sr(-1) above 10(17) eV and decreases roughly as E (-2), making this diffuse flux the dominant one from cosmic-ray interactions for energy thresholds between 10(17) and 10(18) 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 similar or equal to 10(18) 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.