Sensitivity of cosmic-ray experiments to ultrahigh-energy photons: Reconstruction of the spectrum and limits on the superheavy dark matter

Abstract

We estimate the sensitivity of various experiments detecting ultrahigh-energy cosmic rays to primary photons with energies above ${10}^{19}\text{ }\text{ }\mathrm{eV}$. We demonstrate that the energy of a primary photon may be significantly (up to a factor of $\ensuremath{\sim}10$) underestimated or overestimated for particular primary energies and arrival directions. We consider distortion of the reconstructed cosmic-ray spectrum for the photonic component. As an example, we use these results to constrain the parameter space of models of superheavy dark matter by means of both the observed spectra and available limits on the photon content. We find that a significant contribution of ultrahigh-energy particles (photons and protons) from decays of superheavy dark matter is allowed by all these constraints.