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The EGRB measured by EGRET can be represented as of the power-law form

Equation 2   (1)

between 0.1 and ~ 50 GeV (statistics limited) [1]. At energies below 30 MeV, the EGRB spectrum appears to be steeper, as determined from an analysis of COMPTEL data [2].

Figure 1, taken from Ref. [3], shows a comparison between the diffuse inner-galactic and extragalactic spectra measured by EGRET. It shows that these diffuse spectra have fundamentally different origins. The galactic spectrum shows evidence of the predicted "bump" from neutral pion decay [4], [5] whereas the extragalactic spectrum shows no such feature as would be expected from cosmic ray p - p interactions. This type of direct spectral information eliminates purely diffuse extragalactic cosmic-ray interaction origin models, such as have been proposed [6] as explanations for the EGRB.

Figure 1

Figure 1. Comparison of Galactic and extragalactic diffuse spectra as determined by EGRET.

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