An Excited Dark Matter Solution to the MeV Galactic Center Excesses

TL;DR

This paper proposes an Excited Dark Matter model that explains the 511 keV line and MeV continuum excesses in the Galactic Center by inelastic scattering and de-excitation of TeV-scale dark matter particles, matching multiple observations.

Contribution

It introduces a minimal XDM model with specific parameters that simultaneously explains the 511 keV line, MeV continuum, and ionization rates in the Galactic Center, unifying these signals.

Findings

Excellent fit to the data with a 1.5 TeV DM particle and 4 MeV mass splitting.

Reproduces the spatial distribution of the 511 keV line and MeV continuum.

Predicts correlated signatures for upcoming MeV gamma-ray missions.

Abstract

Recent COMPTEL data analysis reveals a $\sim$ 2 MeV continuum excess whose spatial distribution closely matches the long-standing 511 keV line observed by INTEGRAL/SPI, indicating a common population of low-energy positrons that is difficult to reconcile with known astrophysical sources or standard thermal dark matter (DM). We show that a minimal Excited Dark Matter (XDM) model naturally explains these features. In this scenario a DM particle $\chi$ is inelastically upscattered into an excited state $\chi^*$, followed by de-excitation $\chi^*\to\chi e^+ e^-$ producing $\sim$2 MeV positrons that reproduce the 511 keV line morphology and the COMPTEL MeV continuum. Using a full cosmic-ray (CR) propagation treatment, we obtain an excellent fit for $m_\chi\simeq$ 1.5 TeV DM particle with mass-splitting $\Delta m =m_{\chi^*}-m_\chi \simeq$ 4 MeV for an inelastic geometric scattering cross section of $\sigma_\textrm{mr}= 3-4\times 10^{-23}\,\textrm{cm}^2$. The same positrons supply a substantial, radially flat contribution to the anomalous Central Molecular Zone (CMZ) ionization rate. This is the first unified treatment of XDM-induced positrons across all three observables, yielding correlated MeV signatures testable by upcoming missions targeting the Galactic MeV band.