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

TL;DR

This paper predicts gamma-ray halos around radio galaxy Cygnus A caused by ultra-high energy cosmic rays interacting with background light, which could be detected by current gamma-ray telescopes, supporting the idea that such galaxies are UHECR sources.

Contribution

It provides the first detailed calculation of gamma-ray halo emission from Cygnus A due to UHECR interactions, suggesting detectability with existing telescopes.

Findings

Gamma-ray halos around Cygnus A are detectable with Fermi.

Ground-based telescopes could observe these halos if UHECRs originate from radio galaxies.

Supports the hypothesis that radio galaxies are sources of UHECRs.

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 ultra-relativistic electrons and positrons that initiate electromagnetic cascades, leading to the production of a gamma-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 gamma-ray telescopes if radio galaxies are the sources of UHECRs.