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

Nearby sources of cosmic rays up to a ZeV(=10²¹ eV) could be observed with a multimessenger approach including secondary γ-rays and neutrinos. If cosmic rays above ~10¹⁸ eV are produced in magnetized environments such as galaxy clusters, the flux of secondary γ-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 ∝E^-2.6. Such sources should be visible at the same time in ultrahigh energy cosmic ray experiments and γ-ray telescopes.