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1.2. EGRET Results (30 MeV to 100 GeV)

At gamma-ray energies above 10 MeV the analysis of diffuse emission is complicated by difficulties in accurately accounting for the Galactic diffuse emission. The following approach is adopted for the EGRET data. The observed intensity Io is assumed to be made up of a Galactic, Ig, and an extragalactic component, Ieg. The extragalactic intensity is then derived by a straight line fit to a plot of observed emission versus model prediction of Galactic emission (Sreekumar et al. 1998). The spectrum is determined to be consistent with a single power law of index (2.1 ± 0.03). The intensity above 100 MeV is (1.45 ± 0.05) · 10-5 cm-2 sec-1 sr-1, which may include some unaccounted extended Galactic diffuse emission. The spectrum derived by Sreekumar et al. (1998) is also shown in Figure 1.

Earlier, independent analysis yielded a power law index (2.11 ± 0.05) (Osborne et al. 1994), or an index (2.15 ± 0.06) and an integrated intensity above 100 MeV of (1.24 ± 0.06) · 10-5 cm-2 sec-1 sr-1 (Chen et al. 1996), which are perfectly consistent with Sreekumar's result in the power law index, but indicate systematic uncertainties in the derivation of the absolute intensity level.