TGRS OBSERVATION OF THE GALACTIC-CENTER ANNIHILATION LINE

The Transient Gamma-Ray Spectrometer (TGRS) experiment is a high-resol ution germanium detector launched on the WIND satellite on 1994 Novemb er 1. Although primarily intended to study gamma-ray bursts and solar flares, TGRS also has the capability of studying slower transients (e. g., X-ray novae) and certain steady sources. We present here results o n the narrow 511 keV annihilation line from the general direction of t he Galactic center accumulated over the period 1995 January through 19 95 October. These results were obtained from the TGRS occultation mode , in which a lead absorber occults the Galactic center region for 1/4 of each spacecraft rotation, thus chopping the 511 keV signal. The occ ulted region is a band in the sky of width 16 degrees that passes thro ugh the Galactic center. We detect the narrow annihilation line from t he Galactic center with flux = (1.64 +/- 0.09) x 10(-3) photons cm(-2) s(-1). The data are consistent with a single point source at the Gala ctic center, but a distributed source of extent up to similar to 30 de grees cannot be ruled out. No evidence for temporal variability on tim escales longer than 1 month was found.