Positron Annihilation in the Nuclear Outflows of the Milky Way

TL;DR

This paper investigates the origin of positrons in the Milky Way's Galactic bulge, analyzing whether nuclear outflows can explain the observed gamma-ray emission, and finds that they cannot simultaneously match the observed morphology and annihilation conditions.

Contribution

It provides a detailed analysis showing that nuclear outflows alone cannot account for the observed positron annihilation distribution in the Milky Way.

Findings

Advective transport in outflows cannot reproduce the observed positron distribution.

Positron annihilation predominantly occurs after cooling to 10^4 K.

Nuclear outflows are unlikely the sole source of Galactic positrons.

Abstract

Observations of soft gamma rays emanating from the Milky Way from SPI/\textit{INTEGRAL} reveal the annihilation of $\sim2\times10^{43}$ positrons every second in the Galactic bulge. The origin of these positrons, which annihilate to produce a prominent emission line centered at 511 keV, has remained mysterious since their discovery almost 50 years ago. A plausible origin for the positrons is in association with the intense star formation ongoing in the Galactic center. Moreover, there is strong evidence for a nuclear outflow in the Milky Way. We find that advective transport and subsequent annihilation of positrons in such an outflow cannot simultaneously replicate the observed morphology of positron annihilation in the Galactic bulge and satisfy the requirement that $90$ per cent of positrons annihilate once the outflow has cooled to $10^4\,\mathrm{K}$.