EGRET Observations of the high latitude diffuse radiation

The all-sky survey in high energy gamma-rays (E > 30 MeV) carried out with the Energetic Gamma Ray Experiment Telescope (EGRET) on the Campion Gamma Ray Observatory has allowed the first study of the extragalactic diffuse gamma-ray emission since the SAS-2 experiment in 1972-73 (Fichtel et al. 1977; Fichtel et al. 1978). The study reported here covers observations made during the period 16 May 1991 to 4 October 1994. The instrument was designed from the beginning with this as a major objective. Most important is that the instrument background must be kept to a minimum. The amount of material in the path of incoming gamma rays was minimized and the orbit was chosen with these objectives in mind by minimizing the exposure to energetic cosmic rays. Energetic interactions of these high energy particles with the material in an around the instrument is an unwanted background source. The low-inclination low-altitude orbit provided by the Shuttle was a good match to these objectives. The all-sky survey reveals a strong galactic emission that extends well away from the galactic plane making the selection of the region used for analysis critical. We have restricted our analysis to regions at latitudes greater than 30 degrees (absolute). The spectrum obtained is consistent with a power-law distribution in energy with a slope of -2.07 ±0.03 and an integrated intensity above 100 MeV of 1.5 10~5 cm~2sr~1sec~1.