LIMITS ON A VARIABLE SOURCE OF 511 KEV ANNIHILATION RADIATION NEAR THE GALACTIC-CENTER

The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission satellite (SMM) has observed a strong Galactic source of 511 keV annihilation radiation from its launch in 1980 to its reentry in 1989. These observations were made during December transits through the instrument's 130° (FWHM) aperture by the Galactic center and are consistent with an extended source having an intensity of ∼2 × 10-3γ cm-2 s-1 averaged over the central radian of Galactic longitude. We have searched these data for evidence of the variable Galactic center source of 511 keV line radiation which was reported to have reappeared in 1988 by Leventhal et al. The SMM data are consistent with, but do not require, a compact source emitting a time-averaged flux of ∼4 × 10-4γ cm-2 s-1 during ∼3 month transits in 1987 and 1988; they are inconsistent (99% confidence level) with a compact source flux in excess of 8 × 10-4γ cm-2 s-1 for each year. We note that the significance of detection of the compact source by previous experiments is strongly dependent on the strength and spatial distribution of the extended Galactic source.