A possible galactic positron annihilation medium - Neutral hydrogen

Previous investigators have ruled out neutral hydrogen as a possible medium for prompt annihilation of galactic positrons principally because the expected two-photon positronium spectrum is wider than the observed line width. 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 line shape. Detailed fits to the galactic center annihilation spectra are consistent with prompt positron annihilation in a neutral hydrogen medium.