Possible galactic positron annihilation medium: Neutral atomic hydrogen.

Previous investigators have ruled out neutral atomic H as a possible medium for prompt annihilation of galactic positrons principally because the expected two-photon positronium spectrum is wider than the observed linewidth. The small fraction of incident positrons that directly annihilate and produce a narrow line was previously thought to be insignificant. Though the fraction is small it is important in determining the lineshape. Detailed fits to the galactic center annihilation spectra are consistent with prompt positron annihilation in a neutral H medium.