Abstract :
Positron annihilation radiation from the galactic center region was first detected nearly 30 years ago. Several potential sources may contribute to this emission, including: (1) β+-decay of nuclei produced in explosive objects (supernovae and novae); (2) γ-γ pair production associated with the accretion of matter onto galactic black holes; (3) positrons produced in pair cascades in pulsar magnetospheres; (4) β+-decay of radioactive nuclei synthesized in Wolf-Rayet and AGB stars and carried in their stellar winds; and (5) positrons produced by interactions of cosmic rays with the interstellar medium. Early evidence for variability of the galactic center emission suggested that one or more discrete sources dominated the emission. However, more recent SMM/GRS, COMPTON/OSSE and WIND/TGRS results indicate that the dominant emission is steady and of diffuse origin. Transient positron production associated with galactic black hole candidates has been reported; however, there are limits for at least one of these events which are in disagreement with the reports. The OSSE instrument has provided the first maps of the emission, and the discovery of a surprising excess at positive galactic latitudes above the center of the Galaxy. The history of these observations is discussed, along with the latest data available from OSSE and TGRS. We also discuss the implications of the observations on our understanding of galactic nucleosynthesis, galactic distributions of supernovae, and the study of the interstellar medium.
