Upper limit to the diffuse cosmic ultrahigh-energyγspectrum

An upper limit to the diffuse cosmic ultrahigh-energy (UHE) $\gamma$ spectrum from 5 TeV up to 1 PeV is inferred from data on the spectrum of the electromagnetic component of secondary cosmic rays recorded by emulsion-chamber experiments at various atmospheric depths. Our Monte Carlo simulation of the atmospheric propagation and interaction of the photons and protons with primary energies of (5×${10}^{12}$)-${10}^{15}$ eV shows that the intensity of uncorrelated $\gamma$ events with visible energies $E_{\gamma }=5-100\mathrm{}$ TeV observed in emulsion-chamber experiments is sensitive to the diffuse cosmic UHE $\gamma$ rays at the level of ∼${10}^{-3\mathrm{}}$ compared to primary cosmic rays in the same energy region. We also discuss the relevance of the UHE $\gamma$ spectrum to topics such as diffuse photons as by-products of the energy loss of extragalactic cosmic rays interacting with the cosmic 2.7 K background radiation, direct photons as a result of possible new production mechanisms, and photons as products of radiative decay of unstable cosmological relic neutrinos of mass ∼5 TeV-1 PeV.