The diffuse flux of energetic extragalactic gamma rays

An improved determination is made of the energy spectrum of the so-called diffuse component of extragalactic gamma rays over the range 30 MeV-10 GeV. The data used comprise gamma ray spectra from the EGRET instrument on the Compton Gamma Ray Observatory and a variety of information on the column densities of gas in various forms (atomic and molecular hydrogen and ionized material) together with model predictions of the contribution from inverse Compton interactions, all the latter being necessary in order to evaluate the corrections to be applied for gamma rays of Galactic origin. Deleting only one known extragalactic source (the gamma -bright quasar 3C 279) the differential intensity is given by I(Egamma )=9.6*10-7 E- gamma cm-2 s-1 sr-1 GeV-1 with gamma =2.11+or-0.05 above 50 MeV. At lower energies there is some evidence for a somewhat lower intensity. The corresponding energy density in extragalactic space is 1.9*10-6 eV cm-3 in the range 100 MeV-10 GeV. The significance of the measured flux is examined.