Interacting dark matter contribution to the Galactic 511 keV gamma ray emission: constraining the morphology with INTEGRAL/SPI observations

TL;DR

This study analyzes the spatial distribution of the 511 keV gamma-ray excess in the Milky Way, testing dark matter models that produce positrons, and finds that certain dark matter profiles fit the observed data well.

Contribution

It compares dark matter scattering models to INTEGRAL/SPI data, constrains the dark matter halo profile, and improves the fit to the gamma-ray morphology with fewer parameters.

Findings

Einasto profile fits the data better than NFW.

Decaying dark matter scenarios are excluded due to broad spatial distribution.

The bulge-to-disk flux ratio is between 1.9 and 2.4.

Abstract

We compare the full-sky morphology of the 511 keV gamma ray excess measured by the INTEGRAL/SPI experiment to predictions of models based on dark matter (DM) scatterings that produce low-energy positrons: either MeV-scale DM that annihilates directly into e+e- pairs, or heavy DM that inelastically scatters into an excited state (XDM) followed by decay into e+e- and the ground state.By direct comparison to the data, we find that such explanations are consistent with dark matter halo profiles predicted by numerical many-body simulations for a Milky Way-like galaxy. Our results favor an Einasto profile over the cuspier NFW distribution and exclude decaying dark matter scenarios whose predicted spatial distribution is too broad. We obtain a good fit to the shape of the signal using six fewer degrees of freedom than previous empirical fits to the 511 keV data. We find that the ratio of flux at Earth from the galactic bulge to that of the disk is between 1.9 and 2.4, taking into account that 73% of the disk contribution may be attributed to the beta decay of radioactive 26Al.