TGRS Observation of the Galactic Center Annihilation Line

The Transient Gamma-Ray Spectrometer (TGRS) experiment is a high-resolution germanium detector launched on the WIND satellite on 1994 November 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 on the narrow 511 keV annihilation line from the general direction of the Galactic center accumulated over the period 1995 January through 1995 October. These results were obtained from the TGRS occultation mode, in which a lead absorber occults the Galactic center region for of each spacecraft rotation, thus chopping the 511 keV signal. The occulted region is a band in the sky of width 16° that passes through the Galactic center. We detect the narrow annihilation line from the Galactic center with flux = (1.64 ± 0.09) × 10-3 photons cm-2 s-1. The data are consistent with a single point source at the Galactic center, but a distributed source of extent up to ~30° cannot be ruled out. No evidence for temporal variability on timescales longer than 1 month was found.