The Gamma-Ray Background Constrains the Origins of the Radio and X-Ray Backgrounds

TL;DR

This paper uses the extragalactic gamma-ray background to constrain the contribution of secondary electrons and positrons to cosmic radio, X-ray, and gamma-ray backgrounds, revealing limits on their origins and associated magnetic fields.

Contribution

It provides new constraints on secondary e+/- contributions to cosmic backgrounds based on gamma-ray observations, linking them to magnetic field strengths and source redshifts.

Findings

IC-upscattered secondary light is less than 25% of the gamma-ray background.

Secondary e+/- contribution to the X-ray background is less than 10%.

Inconsistencies arise with the ARCADE radio excess unless magnetic fields are milliGauss or higher.

Abstract

Cosmic ray protons generate gamma-rays, neutrinos, and secondary electrons and positrons (e+/-) through pion-producing collisions with gas atoms. Any synchrotron or Inverse Compton (IC) radiation from secondary e+/- is therefore accompanied by pionic gamma-rays. Using the extragalactic gamma-ray background, we constrain the contribution of secondary e+/- to the cosmic radio, X-ray, and soft gamma-ray backgrounds. These bounds depend on the unknown hadronic contribution to the gamma-ray background and the backgrounds' source redshifts. With our assumptions, we find that IC-upscattered light from secondaries is <~ 1/4 of the MeV - GeV gamma-ray background and <~ 10% of the 0.5 keV - 1 MeV background (for sources at a redshift z <~ 10). The low intensity of the observed gamma-ray background is marginally inconsistent with a secondary e+/- origin for the radio background reported by ARCADE at ~3 GHz, unless the magnetic field strength in their sources is milliGauss or greater. These limits on the magnetic field strength are sensitive to uncertainties. However, any contribution to the gamma-ray background from sources not responsible for the ARCADE excess increases the inconsistency.