Positron Transport And Annihilation In The Galactic Bulge

TL;DR

This paper reviews models of positron propagation and annihilation in the Milky Way's bulge, aiming to understand the origin of observed gamma-ray signals and the role of positron transport in different interstellar medium phases.

Contribution

It provides a comprehensive discussion of existing models for positron transport and annihilation in the Galactic bulge, highlighting the importance of propagation effects in interpreting observations.

Findings

Positron annihilation gamma-ray morphology constrains source locations.

Propagation models influence the interpretation of positron origins.

Understanding ISM phases is crucial for accurate positron transport modeling.

Abstract

The annihilation of positrons in the Milky Way galaxy has been observed for $\sim 50$ years however the production sites of these positrons remains hard to identify. The observed morphology of positron annihilation gamma-rays provides information on the annihilation sites of these Galactic positrons. It is understood that the positrons responsible for the annihilation signal originate at MeV energies. The majority of sources of MeV positrons occupy the thin, star forming disk of the Milky Way. If positrons propagate far from their sources, we must develop accurate models of positron propagation through all ISM phases in order to reveal the currently uncertain origin of these Galactic positrons. On the other hand, if positrons annihilate close to their sources, an alternative source of MeV positrons with a distribution that matches the annihilation morphology must be identified. In this work, I discuss the various models that have been developed to understand the origin of the $511\,\mathrm{keV}$ line from the direction of the Galactic bulge, and the propagation of positrons in the ISM.