Extragalactic gamma-ray emission: CGRO results

The first all-sky survey above 1 MeV by the COMPTEL and EGRET instruments on board the Compton Gamma Ray Observatory has provided new results on the extragalactic diffuse emission. The extragalactic emission over the energy range 1 MeV–100 GeV is well described by a broken power law spectrum, the spectrum being harder above 30 MeV. 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 emission, is that it arises primarily from unresolved γ-ray-emitting blazars at least above 10 MeV. The consistency of the average γ-ray blazar spectrum with the derived extragalactic diffuse spectrum strongly argues in favor of such an origin. The origin of the emission in the 1–30 MeV remains less well understood. Additional contributions from sources with jets not necessarily aligned close to our line-of-sight are also examined. Future gamma-ray missions such as GLAST with enhanced spectral capabilities, are expected to provide the next major advances in resolving the origin of the extragalactic γ-ray background.