Cosmogenic gamma-rays and the composition of cosmic rays
  We discuss the prospects of detecting the sources of ultra-high energy (UHE)
cosmic ray (CR) nuclei via their emission of cosmogenic gamma-rays in the GeV
to TeV energy range. These 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 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 (~ A) and Bethe-Heitler
pair production (~ Z^2). We show that the cosmogenic 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.
