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An estimate of the expected photon flux above $10^{17}~$eV from theinteractions of ultra-high energy cosmic rays with the matter in the Galacticdisk is presented. Uncertainties arising from the distribution of the gas inthe disk, the absolute level of the cosmic ray flux, and the composition of thecosmic rays are taken into account. Within these uncertainties, the integratedphoton flux above $10^{17}~$eV is, averaged out over Galactic latitude lessthan $5^\circ$, between $\simeq3.2{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$ and $\simeq8.7{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$. The all-sky average valueamounts to $\simeq 1.1{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$ above$10^{17}~$eV and decreases roughly as $E^{-2}$, making this diffuse flux thedominant one from cosmic-ray interactions for energy thresholds between$10^{17}~$eV and $10^{18}$~eV. Compared to the current sensitivities ofdetection techniques, a gain between two and three orders of magnitude inexposure is required for a detection below $\simeq 10^{18}$~eV. Theimplications for searches for photon fluxes from the Galactic center that wouldbe 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 belowwhich other signals would be overwhelmed.

astrophysical journal/the astrophysical journal

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