The Positron Puzzle

Abstract

The Positron Puzzle is a half-century old conundrum about the origin of the Galactic γ-ray emission line at photon energies of 511 keV, and the shape of its morphology, showing a bulge-to-disk luminosity ratio of ∼ 1 -unlike any astrophysical source distribution. Positrons (e + s) that have been cooled to the eV scale capture electrons (e − s) and form the intermediate bound state of Positronium (Ps) which decays on a nano-second timescale into two or three photons. Assuming the emission to originate from the Galactic bulge, centre, and disk, a visible annihilation rate in the Milky Way of ∼ 5 × 10 43 e + s −1 has to be explained, either by a quasi-steady state of production and annihilation, or by possibly multiple burst-like events that flood the Galaxy with e + s, then fading away on a Myr timescale. In this paper, I will review what the real Positron Puzzle is, where data and simulations have been used inadequately which resulted in false claims and an apparent quandary, what we really know and absolutely not know about the topic, and how this epistemic problem might be advancing.