Observations of gamma-ray line emission from the Galactic Center region

A series of observations of the Galactic Center 511 keV electron-positron annihilation line spanning a decade has begun to severely constrain theoretical ideas concerning the nature of the source. The fluxes reported range from (4.18+/-1.56)+/-10-3γ/cm2-s to less than 0.7+/-10-3γ/cm2-s (2σ upper limit). Data from high-resolution germanium spectrometers show a narrow (1.6 keV+0.9,-1.6 keV), unredshifted (E = 510.9+/-0.25 keV) emission feature which varies in intensity by a factor of up to three, in six months or less. The combined results show that, in addition to any possible spatially extended component, a significant fraction of the emission must come from one or a few sources with size r<~1018cm, temperature T<~105 K, and typical luminosity ~=1037 ergs s-1. Simple models indicate the mass M of any compact central object is M < 109Msolar. There is a strong need for continued monitoring of the variations of this source.