Observations of annihilation radiation from the Galactic center region

The experiments that have demonstrated variability of the 511 keV positron annihilation line from the Galactic center region are reviewed. The simplest history consistent with all (but one) of the eleven measurements is that of a narrow (<2.5 keV FWHM), unredshifted (dE<.25 keV) line, which was constant in intensity ((1.6+/-0.1)×10-3 ph cm-2 s-1 at earth) from before 1970 until the beginning of 1980, when it dropped to zero in less than a year. The limited evidence for detection of the continuum expected from the three-photon decay of orthopositronium is evaluated, with particular attention to the possibility of simulation by atmospheric scattering. The question of possible correlation between variations in the inverse-power-law Galactic center continuum and the strength of the 511 keV annihilation line is discussed; the evidence is inconclusive.