GZK Photons Above 10 EeV

TL;DR

This paper calculates the flux of GZK photons produced by extragalactic cosmic rays, considering various source and background parameters, and extends the analysis to nuclei, providing estimates of photon fractions above 10 EeV.

Contribution

First calculation of GZK photon flux produced by nuclei, expanding the understanding of ultra-high-energy cosmic ray interactions and their observable photon signatures.

Findings

GZK photon fraction could be up to a few percent above 10 EeV for nucleons.

Photon fraction for nuclei is smaller, with a maximum 2-3 times less than for nucleons.

Estimated GZK photon fractions vary widely depending on source spectra and backgrounds.

Abstract

We calculate the flux of "GZK-photons", namely the flux of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so called GZK effect. This flux depends on the UHECR spectrum on Earth, of the spectrum of nucleons emitted at the sources, which we characterize by its slope and maximum energy, on the distribution of sources and on the intervening cosmological backgrounds, in particular the magnetic field and radio backgrounds. For the first time we calculate the GZK photons produced by nuclei. We calculate the possible range of the GZK photon fraction of the total UHECR flux for the AGASA and the HiRes spectra. We find that for nucleons produced at the sources it could be as large as a few % and as low as 10^{-4} above 10 EeV. For nuclei produced at the sources the maximum photon fraction is a factor of 2 to 3 times smaller above 10 EeV but the minimum could be much smaller than for nucleons. We also comment on cosmogenic neutrino fluxes.