The Positron Puzzle

TL;DR

The Positron Puzzle concerns the mysterious origin and distribution of positron annihilation signals in the Milky Way, with ongoing debates about whether steady or burst-like sources explain the observed gamma-ray emission.

Contribution

This paper critically reviews the existing data, simulations, and interpretations to clarify misconceptions and outline what is truly known about the Galactic positron annihilation phenomenon.

Findings

Identifies inadequacies in previous data and simulation analyses.

Clarifies misconceptions leading to the apparent quandary.

Highlights what is genuinely understood and unknown about the Positron Puzzle.

Abstract

The Positron Puzzle is a half-century old conundrum about the origin of the Galactic $\gamma$-ray emission line at photon energies of 511 keV, and the shape of its morphology, showing a bulge-to-disk luminosity ratio of $\sim 1$ - unlike any astrophysical source distribution. Positrons that have been cooled to the eV scale capture electrons 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 $\sim 5 \times 10^{43}\,\mathrm{e^+\,s^{-1}}$ 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 positrons, 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.