Positron annihilation in the interstellar medium

We study positronium formation and positron annihilation in a model for the interstellar medium consisting of cold cloud cores, warm, partially ionized cloud envelopes, and hot intercloud gas. We calculate the gamma-ray spectra resulting from positron annihilation in these components of the interstellar medium. We then combine the spectra from the individual components using two limiting assumptions for the propagation of the positrons: (1) the positrons propagate freely throughout the interstellar medium, and (2) the positrons are excluded from the cold cloud cores. In the first case, the bulk of the positrons annihilate in the cloud cores and the annihilation line exhibits broad wings resulting from the annihilation of positronium formed by charge exchange in flight. In the second case, the positrons annihilate mainly in the warm envelopes, and the line wings are suppressed. Positrons could be prevented from penetrating into the cores by magnetic fields. The fraction f of positrons annihilating via positronium is ≃0.9 for the first model, independent of the amount of dust in the interstellar medium. This fraction is also ≃0.9 for the second model, provided the amount of dust is not large. The comparison of this value with observations suggests that for the compact object near the Galactic center, whose existence is implied by observations of time-variable 511 keV line emission, f is lower than the value calculated for the interstellar component. In the compact source a low f could result from annihilation in a hot (≳5 × 105 K) gas or in a dusty, partially ionized medium.