2022ApJ...929...55B

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 x 10^–2 km^–2 yr^–1 sr^–1 and ≃8.7 x 10^–2 km^–2 yr^–1 sr^–1. The all-sky average value amounts to ≃1.1 x10^–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.