Positron Annihilation in the Milky Way and beyond

TL;DR

This paper investigates the origin of the 511 keV gamma-ray signal in the Milky Way, testing dark matter hypotheses by observing satellite galaxies, and finds evidence against dark matter as the sole source, with one galaxy showing alternative emission.

Contribution

The study provides observational constraints on dark matter explanations for positron annihilation signals by analyzing satellite galaxies with INTEGRAL/SPI, identifying a non-dark matter source in Reticulum II.

Findings

No significant 511 keV signals in most satellite galaxies.

Reticulum II shows strong annihilation emission, likely from a microquasar.

Results challenge dark matter as the primary source of galactic positrons.

Abstract

The electron-positron annihilation gamma-ray signal at 511 keV in the Milky Way is investigated towards a possible dark matter interpretation. If all bulge positrons were created by dark matter particle annihilation, the satellite galaxies of the Milky Way, apparently being dominated by dark matter, should also show measurable 511 keV signals. Using INTEGRAL/SPI, we test for emission in 39 neighbouring dwarf satellite galaxies, and found a consistent trend against a dark matter scenario. One galaxy, Reticulum II, shows up as a strong source of annihilation emission, which we interpret as the presence of a microquasar, ejecting pair-plasma into the galaxy's interstellar medium.