1999ApJ...520..196D

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)x10^–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)/W_CO=(1.35±0.15)x10²⁰ cm^–2 (K km.s^–1)^–1.