EGRET Observations of the North Galactic Pole Region

Abstract

The EGRET instrument aboard the Compton Gamma Ray Observatory (CGRO) has carried out the first ever all-sky survey in high-energy gamma rays. Initial analysis of the region near the north Galactic pole showed the presence of several previously undetected point sources, many of which exhibit time variability in the observed emission. A more detailed analysis using all observations during phases I and II of the CGRO observing plan shows a total of 21 point sources detected above a 4 σ significance level in the region of the sky with b > 40°. Results on source location, possible identification, energy spectrum, and time variability are discussed. Of the 21 sources, 17 are identified as belonging to the blazar class of active galactic nuclei. They include 0917+449, 1127-145, and 1222+216, which are positionally coincident with previous unidentified EGRET sources. The four remaining unidentified sources appear to have no known counterparts. No evidence is found for high-energy gamma-ray emission from M82, M87, or the Coma Cluster. The upper limit from Coma is used to derive a lower limit of 0.4 μG on the intracluster magnetic field. © 1996. The American Astronomical Society. All rights reserved.