A Possible High-Energy Gamma-Ray Source at the Galactic Center

In the course of a full-sky survey, the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory has detected a high-energy (30 MeV - 30 GeV) gamma -ray excess which is consistent in position with the galactic center (within 0.4 degrees of l=-0.1, b=+0.2, with 95% confidence). Thus, the low-energy gamma -ray source 1E1740.7-2942 is excluded with moderately high confidence. There is weak evidence of variability. The flux during the initial exposure (7/12/91-7/26/91) was 0.72+/-0.13 *E(-6) cm(-2) s(-1) . During a subsequent exposure (12/12/91-12/27/91) it was 1.38+/-0.20 *E(-6) cm(-2) s(-1) , corresponding to a high-energy gamma -ray luminosity of ~6*E(36) ergs/s if it is at the galactic center. The excess is observed to be distributed as the EGRET point spread function, which is narrower than the distribution of the wide-line molecular clouds at galactic radius ~400 pc. A diffuse origin at galactic radius <400 pc has not been excluded. If subsequent observations establish variability, then the gamma -ray emission is possibly due to the black hole candidate Sgr A(*) .