Predicting the UHE photon flux from GZK-interactions of hadronic cosmic rays using CRPropa 3

Abstract

The spectrum of ultra high energy (UHE) cosmic rays as measured by the Pierre Auger Observatory indicates a strong flux suppression above 50 EeV. The origin of this suppression is still unclear. One possible explanation is the Greisen-Zatsepin-Kuzmin (GZK) process, in which UHE cosmic rays interact with the cosmic microwave background. Indirect evidence for the GZK-process could be provided by the search for UHE photons produced in such an interaction. A signal of UHE photons could not yet be identified among the cosmic rays. Hence, upper limits on the UHE photon flux have been derived from experimental data of various experiments. In order to interpret these limits, theoretical predictions are needed.In this contribution, new predictions on the UHE photon flux above 10^15.8 eV are derived assuming different compositions of the initial cosmic rays. The simulation study has been done using CRPropa 3. Latest results regarding the extragalactic medium and the mass composition of cosmic rays as measured by the Pierre Auger Observatory are taken into account. For all compositions, the predictions stay below the current upper limits on the UHE photon flux derived from experimental data. The main uncertainties on the predictions originate from the lack of knowledge about the initial source spectra of UHE cosmic rays.