The Extragalactic Diffuse Gamma-Ray Emission

The all-sky surveys in γ-rays by the imaging Compton telescope (COMPTEL) and the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory for the first time allows detailed studies of the extragalactic diffuse emission at γ-ray energies greater 1 MeV. A preliminary analysis of COMPTEL data indicates a significant decrease in the level of the derived cosmic diffuse emission from previous estimates in the 1–30 MeV range, with no evidence for an MeV-excess, at least not at the levels claimed previously. The 1–30 MeV flux measurements are compatible with power-law extrapolation from lower and higher energies. These new results indicate that the possible contributions to the extragalactic emission from processes that could explain the MeV-excess, such as matter-antimatter annihilation, is significantly reduced. At high energies (> 30 MeV), the extragalactic emission is well described by a power law photon spectrum with an index of –(2.10±0.03) in the 30 MeV to 100 GeV energy range. No large scale spatial anisotropy or changes in the energy spectrum are observed in the deduced extragalactic emission. The most likely explanation for the origin of this extragalactic γ-ray emission above 10 MeV, is that it arises primarily from unresolved γ-ray-emitting blazars. The consistency of the average γ-ray blazar spectrum with the derived extragalactic diffuse spectrum strongly argues in favor of such an origin. The extension of the power law spectrum to 100 GeV implies the average emission from γ-ray blazars extends to 100 GeV.