GRIS DETECTIONS OF THE 511 KEV LINE FROM THE GALACTIC-CENTER REGION IN 1992

The Gamma Ray Imaging Spectrometer (GRIS) was flown on balloons over Alice Springs, Australia on 1992 April 26 and May 7. A full Galactic center transit (approximately 12 hr) was achieved on both flights with the instrument working normally. The electron/positron annihilation line was detected on both flights. The line fluxes and line widths were found to be (7.7 +/- 1.2) x 10(-4) and (8.9 +/- 1.1) x 10(-4) photons cm-2 s-1 and 1.3 +/- 0.7 and 3.6 +/- 1.0 keV, respectively. These results are compared to each other and earlier (1988) GRIS results to produce suggestive evidence for source variability. Near-contemporaneous OSSE/CGRO Galactic center observations indicate that the GRIS results cannot be due solely to a single point source like 1E 1740.7-2942 within a few degrees of the Galactic center. The GRIS 1992 results represent the first time that successive high-resolution balloon measurements have been achieved on a time scale of days.