Spectral shape and photon fraction as signatures of the Greisen-Zatsepin-Kuzmin cutoff

Abstract

With the prospect of measuring the fraction of arriving secondary photons, produced through photo-pion energy-loss interactions of ultra-high-energy cosmic ray (UHECR) protons with the microwave background during propagation, we investigate how information about the local UHECR source distribution can be inferred from the primary (proton) to secondary (photon) ratio. As an aid to achieve this, we develop an analytic description for both particle populations as a function of propagation time. Through a consideration of the shape of the Greisen-Zatsepin-Kuzmin cutoff and the corresponding photon fraction curve, we investigate the different results expected for both different maximum proton energies injected by the sources, as well as a change in the local source distribution following a perturbative deformation away from a homogeneous description. At the end of the paper, consideration is made as to how these results are modified through extragalactic magnetic field effects on the proton's propagation. The paper aims to demonstrate how the shape of the cosmic ray flux in the cutoff region, along with the photon fraction, are useful indicators of the cutoff origin as well as the local UHECR source distribution.