GALACTIC 0.511 MEV LINE EMISSION

Observations over two decades with various balloon-borne instruments a nd instruments on HEAO 3, SMM, and CGRO have revealed the existence of strong 0.511 MeV line emission from the direction of the Galactic cen ter. We have attempted to fit the spatial distribution of this emissio n with distributions of various Galactic constituents, but found that none of these distributions alone could fit the data. Of the assumed d istributions, those of Galactic novae and hot (10(8) K) plasma are the most strongly peaked at the Galactic center. But we found that the 0. 511 MeV line emission is even more strongly peaked. This new result is the direct consequence of the analysis of recent data obtained with t he OSSE instrument on CGRO. We have thus considered two-component mode ls consisting of an enhanced central Galactic component superposed on a Galactic plane component given by any one of our previously assumed distributions. For the central Galactic component we take either a sin gle point source at the location of the source of annihilation radiati on 1E 1740.7-2942 or an extended spheroidal distribution of several hu ndred parsecs in size. We find acceptable fits in both cases. We sugge st that this additional component of the emission from the Galactic ce nter region is due to positrons produced near one or more black holes. Such a component was also suggested by the variability of the 0.511 M eV line flux observed with balloon-borne high-resolution detectors. To produce the very narrow line observed by these high-resolution detect ors at almost precisely 0.511 MeV the positrons must escape from the v icinity of the holes and annihilate in the surrounding interstellar me dium. We have reexamined the issue of the time variability of the 0.51 1 MeV line emission. Although the significance of the variability impl ied by all of the earlier observations is diminished by a large number of recent OSSE observations which show no variability, the latter mea surements merely indicate that the flux remained relatively constant o nly over a period of about 1 year (1991-1992). Positron production in the Galactic center region by a small number of variable sources would lead to variable 0.511 MeV line emission. The timescale of the variat ion depends on the density of the gas in which the positrons annihilat e. It could be as short as several months if the annihilation is in de nse molecular clouds, but would be much longer and essentially undetec table if the positrons annihilate in lower density interstellar gas.