THE y -RAY BACKGROUND CONSTRAINS THE ORIGINS OF THE RADIO AND X-RAY BACKGROUNDS

摘要

Cosmic ray protons generate γ-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 γ-rays. Using the extragalactic γ-ray background, we constrain the contribution of secondary e± to the cosmic radio, X-ray, and soft γ-ray backgrounds. These bounds depend on the unknown hadronic contribution to the γ-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 γ-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 y -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 y -ray background from sources not responsible for the ARCADE excess increases the inconsistency.