Gamma Rays from Ultra-High-Energy Cosmic Rays in Cygnus A

Abstract

Ultra-high-energy cosmic rays (UHECRs) accelerated in the jets of active galactic nuclei can accumulate in high magnetic field, ~100 kpc-scale regions surrounding powerful radio galaxies. Photohadronic processes involving UHECRs and photons of the extragalactic background light make ultrarelativistic electrons and positrons that initiate electromagnetic cascades, leading to the production of a γ-ray synchrotron halo. We calculate the halo emission in the case of Cygnus A and show that it should be detectable with the Fermi Gamma-Ray Space Telescope and possibly detectable with ground-based γ-ray telescopes if radio galaxies are the sources of UHECRs.