A possible Galactic center positron annihilation medium: neutral hydrogen

A series of laboratory experiments in neutral hydrogen has yielded several quantities that may be relevant to the 511 keV positron annihilation line coming from the direction of the Galactic center. The annihilation linewidth was measured for Ps, formed by positrons slowing down from high energies in H2 and He. The fraction of positrons which survive below the threshold for Ps formation was measured with a pulsed beam technique. The linewidth for positrons directly annihilating with bound electrons in H2 was measured with another pulsing technique. The linewidth measurements agreed with theoretical predictions while the survival fraction did not agree. The combined measurements in H2 are used to construct the annihilation spectrum expected for galactic positrons in neutral molecular hydrogen regions. The resulting lineshape was fit directly to the HEAO-3 satellite data. The fits indicate that neutral hydrogen is a possible annihilation medium, contrary to previous reports.

Recent papers discussing positron annihilation data from the Galactic center have used several incompatible definitions of ``the positronium fraction.'' A single definition applied to the data shows that the observational data are consistent with a triplet Ps annihilation spectrum included at a level of nearly 100% Ps formation. This is also consistent with a neutral hydrogen medium and with other models such as an ionized or cool dusty ionized region.