Propagation of extragalactic photons at ultra-high energy with the EleCa code

Abstract

Ultra-high energy (UHE) photons play an important role as an independent probe of the photo-pion production mechanism by UHE cosmic rays. Their observation, or non-observation, may constrain astrophysical scenarios for the origin of UHECRs and help to understand the nature of the flux suppression observed by several experiments at energies above 1019.5eV. The interaction length of UHE photons above 1017eV ranges from a few hundred kpc up to tens of Mpc. Interactions with the extragalactic background radiation initiate the development of electromagnetic cascades which affect the fluxes of photons observed at Earth. The interpretation of the current experimental results rely on the simulations of the UHE photon propagation. In this paper, we present the novel Monte Carlo code “EleCa” to simulate the Electromagnetic Cascading initiated by high-energy photons and electrons.We provide an estimation of the survival probability for photons inducing electromagnetic cascades as a function of their distance from the observer and we calculate the distances within which we expect to observe UHE photons with energy between 1017 and 1019eV. Furthermore, the flux of GZK photons at Earth is investigated in several astrophysical scenarios where we vary both the injection spectrum and composition, and the intensity of the intervening extragalactic magnetic field. Although the photon propagation depends on several astrophysical factors, our numerical predictions combined with future experimental observations (or non-observations) of UHE photons in the energy range between 1017.5eV and 1020eV can help to constrain these scenarios.