TGRS Observation of the Galactic Center Annihilation Line

The TGRS (Transient Gamma-Ray Spectrometer) experiment is a high-resolutiongermanium detector launched on the WIND satellite on Nov. 1, 1994. Althoughprimarily intended to study gamma-ray bursts and solar flares, TGRS also hasthe capability of studying slower transients (e.g. x-ray novae) and certainsteady sources. We present here results on the narrow 511 keV annihilation linefrom the general direction of the Galactic Center accumulated over the periodJan. 1995 through Oct. 1995. These results were obtained from the TGRSoccultation mode, in which a lead absorber occults the Galactic Center regionfor 1/4 of each spacecraft rotation, thus chopping the 511 keV signal. Theocculted region is a band in the sky of width 16 degrees that passes throughthe Galactic Center. We detect the narrow annihilation line from the galacticcenter with flux = $(1.64\pm0.09)\times10^{-3} {photons} {cm}^{-2} {s}^{-1}$.The data are consistent with a single point source at the galactic center, buta distributed source of extent up to ~30 degrees cannot be ruled out. Noevidence for temporal variability on time scales longer than 1 month was found.Comment: 11 pages + 5 Postscript figure