Transient Gamma-Ray Spectrometer Measurements of the Positron Annihilation Spectrum from the Galactic Center

We have obtained spectra of the Galactic center at energies of 400-600 keV from high-resolution data acquired by the Transient Gamma-Ray Spectrometer Ge detector on board the WIND mission during 1995-1997. The data were obtained using an on-board occulter and are relatively free from systematics and backgrounds. Analysis of the spectra reveals a well-resolved electron-positron annihilation line at 511 keV and the associated continuum due to annihilation via positronium formation. Measurements of the line width and the continuum-to-line ratio allow some constraints to be placed on the interstellar sites where annihilation occurs.