ON THE ORIGIN OF VARIABLE 511 KEV LINE EMISSION FROM THE GALACTIC-CENTER REGION

Variable narrow-line emission at 511 keV, due to positron annihilation, has been observed from the region of the Galactic center for over a decade with high-resolution Ge spectrometers. The variable nature of this emission suggests that a significant fraction of the observed radiation is produced by a single source in the central region of the Galaxy. Recent observations with an imaging gamma-ray spectrometer of low energy resolution have revealed a daylong burst of annihilation radiation from the X-ray source 1E 1740.7-2942 located at an angular distance of 0.9-degrees from the Galactic center and aligned with a dense molecular cloud. We propose that the variable narrow 511 keV line emission is due to positrons released impulsively (time scale approximately 1 day) from 1E 1740.7 - 2942 into the molecular cloud where they slow down and annihilate on a longer time scale of up to a year.