Galactic Positron Annihilation Observations with OSSE

SMM/GRS, CGRO/OSSE and WIND/TGRS results indicate that the dominant 511 keV emission from the galactic center region is steady and of diffuse origin. The OSSE instrument on CGRO has provided the first maps of the 511 keV emission, and the discovery of a surprising excess at positive galactic latitudes above the center of the Galaxy (Purcell et al. 1997b). Most of the annihilation emission is in a 3-photon continuum below 500 keV. Although the continuum signal is stronger, with a positronium fraction near unity, the analysis is more complicated because of continuum contributions to the spectral fits from discrete sources and cosmic ray interactions with the. Kinzer et al. (1999b) report on 1-dimensional latitude and longitude distributions of the positronium continuum emission. Mapping the continuum emission is of importance in order to confirm the features in the 511 keV maps and to study the annihilation sites. We report on OSSE results with an expanded data set, which includes observations from CGRO cycles 7 and 8, inclusion of the positronium continuum emission, and additional pointed observations along the galactic plane not included in earlier analyses.