Positrons in the Galaxy: their births, marriages and deaths

TL;DR

This paper reviews the origins, formation, and annihilation processes of positrons in the galaxy, discussing their sources, how they form positronium, and whether the observed phenomena are interconnected.

Contribution

It provides a comprehensive review of the current understanding of galactic positrons, including their sources, formation of positronium, and annihilation mechanisms.

Findings

Positrons originate from various astrophysical sources.

Positronium formation occurs before annihilation.

The link between positron sources and observed annihilation signals is explored.

Abstract

High energy (~GeV) positrons are seen within cosmic rays and observation of a narrow line at 511 keV shows that positrons are annihilating in the galaxy after slowing down to ~keV energies or less. Our state of knowledge of the origin of these positrons, of the formation of positronium 'atoms', and of the circumstances of their annihilation or escape from the galaxy are reviewed and the question of whether the two phenomena are linked is discussed.