The extragalactic gamma-ray background

The COMPTEL and EGRET detectors aboard the Compton Gamma-Ray Observatory measured an extragalactic γ-ray background (EGRB) extending from ∼1 MeV to ∼100 GeV. Calculations performed making reasonable assumptions indicate that blazars can account for the background between ∼10 MeV and ∼10 GeV. Below 30 MeV, the background flux and spectrum are not very well determined and a dedicated satellite detector will be required to remedy this situation. Below 10 MeV, supernovae and possibly AGN may contribute to the extragalactic background flux. Above 10 GeV, the role of blazars in contributing to the background is unclear because we do not have data on their spectra at these energies and because theoretical models predict that many of them will have spectra which should cut off in this energy range. At these higher energies, a new component, perhaps from topological defects, may contribute to the background, as well as X-ray selected BL Lac objects. GLAST should provide important data on the emission of extragalactic sources above 10 GeV and help resolve this issue. GLAST may also be able to detect the signature of intergalactic absorption by pair production interactions of background γ-rays of energy above ∼20 GeV with starlight photons, this signature being a steepening of the background spectrum.