Positron annihilation in the Galactic Center: Cheshire Cat Compton scattering and excess continuum

Two separate observations of the gamma-ray spectrum originating from the Galactic Center were made by HEAO-3 in the fall of 1979 and in the spring of 1980. The 2-gamma 511 KeV annihilation line flux decreased by a factor of three over the corresponding six month period, whereas the excess gamma-ray continuum below the 511 KeV line, often interpreted as 3-gamma decay of orthopositronium, barely changed. This apparent discrepancy in the temporal behavior makes it difficult to associate the bulk of the excess continuum with the 3-gamma decay of positronium. We will show that Compton scattering of the line and high energy radiation provides a natural explanation for the surprisingly small change seen in the excess continuum.