Cosmogenic gamma rays and the composition of cosmic rays

Abstract

We discuss the prospects of detecting the sources of ultrahigh energy (UHE) cosmic ray (CR) nuclei via their emission of cosmogenic $\ensuremath{\gamma}$ rays in the GeV to TeV energy range. These $\ensuremath{\gamma}$ rays result from electromagnetic cascades initiated by high energy photons, electrons, and positrons that are emitted by CRs during their propagation in the cosmic radiation background and are independent of the simultaneous emission of $\ensuremath{\gamma}$ rays in the vicinity of the source. The corresponding production power by UHE CR nuclei (with mass number $A$ and charge $Z$) is dominated by pion photo production ($\ensuremath{\propto}A$) and Bethe-Heitler pair production ($\ensuremath{\propto}{Z}^{2}$). We show that the cosmogenic $\ensuremath{\gamma}$-ray signal from a single steady UHE CR source is typically more robust with respect to variations of the source composition and injection spectrum than the accompanying signal of cosmogenic neutrinos. We study the diffuse emission from the sum of extragalactic CR sources as well as the point-source emission of the closest sources.