Current status of the Galactic Center positron annihilation source.

A series of 12 balloon and satellite experiments dating back to 1970 and involving six different scientific groups, has established the existence of a powerful, compact, and variable source of 511 keV positron annihilation line radiation within a few degrees of the Galactic Center (GC). Surprisingly, the source was observed to "turn off" rather abruptly at the beginning of the decade. The three previous balloon flights of the joint Bell/Sandia gamma-ray astronomy group's high resolution Ge telescope have played an important role in discovering the source and establishing its variability 2,3,4. We report here the results of our fourth flight which indicate that the source was still an "off" or "low" state in late 1984.