Constraints on Particle Acceleration from the Extragalactic Γ-ray Background

The propagation of γ-rays through metagalactic space is associated with pair creation and subsequent inverse-Compton scattering off low-energy background radiation. As a consequence, γ-rays of very high energy emitted by remote sources are reprocessed into the window from 10 MeV to 30 GeV conserving the injected energy. Any cosmologically distributed population of γ-ray sources therefore contributes to the diffuse γ-ray background in this energy band which is well-determined from recent observations with the Compton Gamma Ray Observatory (CGRO). Since the γ-rays trace accelerated particles, the observed flux of diffuse γ-rays also constrains the global efficiency for particle acceleration. Radio galaxies can account for the γ-ray background if their particle acceleration efficiency considerably exceeds ∼ 18% implying that particle acceleration is an essential part of the thermodynamics in these sources.