The Distribution of Galactic 511 keV Positron Annihilation Radiation

The Oriented Scintillation Spectrometer Experiment OSSE on NASAs Compton Gamma-Ray Observatory has completed numerous observations of the Galactic plane and Galactic center region. A principle objective of these observations was to measure the distribution of the Galactic positron annihilation radiation. Most of these observations provided positive detections of the narrow 511 keV annihilation line. These data were fitted using several diffuse distribution models representing various populations of progenitor objects. The only model investigated which is not rejected is a two-component distribution consisting of spheroidal and disk components all other models investigated can be rejected at the 45 sigma confidence level. The size of the spheroidal component is found to be 500 pc the disk component of the distribution is not well constrained by the current observations. Adding a point source of emission at the location of the black hole candidate 1E 1740.7-2942 does not significantly improve the model fits. There is no evidence for significant time variability of the 511 keV line flux during observations of the Galactic center region. When compared to historical observations of the Galactic center region by other instruments, the two-component diffuse model suggests a limit to a time-variable component of the 511 keV line flux of 4 x 10exp -4 gamma sq cmexp -2 sexp -1.