Galactic Center Positronium Fraction: Observations and Simulations

Recent papers discussing positron annihilation data from the Galactic center have used several incompatible definitions of "the positronium fraction". These differences are sorted out and the observations presented in a consistent manner. The authors find that the apparent positronium (Ps) fraction measured could indicate the presence of Ps at a level consistent with a model for a neutral medium, derived from laboratory experiments, and other models such as an ionized medium or one where dust plays a major role.