Photon/proton ratio as a diagnostic tool for topological defects as the sources of extremely high-energy cosmic rays

Abstract

The hypothesis of topological defects (from grand unified and/or Planck scales) as the sources of extremely high-energy (> ${10}^{18}$ eV) cosmic rays predicts an unusually high content of $\gamma$ rays at energies $E\gtrsim {10}^{20}\mathrm{eV} (\frac{\gamma }{p}\ge 1)$ and $E\lesssim {10}^{14}\mathrm{eV} \left(\frac{\gamma }{p}\gtrsim {10}^{-3\mathrm{}}\right)$. This can be used as a signature for testing the hypothesis in forthcoming experiments.