Secondary gamma rays from ultrahigh energy cosmic rays produced in magnetized environments

Nearby sources of cosmic rays up to a ZeV(=$10^21 eV) could be observed with a multimessenger approach including secondary gamma-rays and neutrinos. If cosmic rays above ~$10^18$ eV are produced in magnetized environments such as galaxy clusters, the flux of secondary gamma-rays can be enhanced by a factor ~10 at Gev energies and by a factor of a few at TeV energies, compared to unmagnetized sources. Particularly enhanced are synchrotron and cascade photons from $e^+e^-$ pairs produced by protons from sources with relatively steep injection spectra [proportional]$E^-2.6.$ Such sources should be visible at the same time in ultrahigh energy cosmic ray experiments and gamma-ray telescopes