EGRET Observations of the Diffuse Gamma‐Ray Emission in Orion: Analysis through Cycle 6

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Abstract
We present a study of the high-energy diffuse emission observed toward Orion by EGRET on the Compton Gamma Ray Observatory. The total exposure by EGRET in this region has increased by more than a factor of 2 since a previous study. A simple model for the diffuse emission adequately fits the data; no significant point sources are detected in the region studied (l = 195° to 220° and b = -25° to -10°) in either the composite data set or in two separate groups of EGRET viewing periods considered. The gamma-ray emissivity in Orion is found to be (1.65 ± 0.11) × 10-26 s-1 sr-1 for E > 100 MeV, and the differential emissivity is well described as a combination of contributions from cosmic-ray electrons and protons with approximately the local density. The molecular mass calibrating ratio is N(H2)/WCO = (1.35 ± 0.15) × 1020 cm-2 (K km s-1)-1.
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