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An Excited Dark Matter Solution to the MeV Galactic Center Excesses

Shyam Balaji, Damon Cleaver, Pedro De la Torre Luque

TL;DR

The paper tackles the Galactic Center puzzles of the 511 keV line, a ~2–3 MeV COMPTEL excess, and anomalous CMZ ionization by proposing a minimal Excited Dark Matter (XDM) model. In this framework, DM inelastically upscatters to a nearly degenerate state, which decays to χ e^+ e^− and injects few-MeV positrons, yielding a bulge-dominated morphology due to a velocity-threshold upscattering $v_{ m th} = \sqrt{4\Delta m/m_χ}$. Using a full cosmic-ray propagation treatment with DRAGON2, the authors show that $m_χ \approx 1.5$ TeV and $Δm \approx 4$ MeV with $σ_{mr} \sim (3-4)\times10^{-23}$ cm^2 can reproduce the INTEGRAL/SPI 511 keV morphology, while simultaneously producing a COMPTEL IfA spectrum (via in-flight annihilation and internal bremsstrahlung) consistent with observations, and contributing a radially flat CMZ ionization rate $ζ_{H_2} \sim 10^{-16}-10^{-15}$ s^{-1}. The scenario remains compatible with unitarity and thermal freeze-out and predicts testable signatures for upcoming MeV-band missions and laboratory searches for a ~10–20 MeV mediator. Overall, the work presents the first unified XDM-based explanation connecting line emission, continuum excess, and CMZ chemistry in the Galactic Center.

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 $χ$ is inelastically upscattered into an excited state $χ^*$, followed by de-excitation $χ^*\toχe^+ e^-$ 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 $m_χ\simeq$ 1.5 TeV DM particle with mass-splitting $Δm =m_{χ^*}-m_χ\simeq$ 4 MeV for an inelastic geometric scattering cross section of $σ_\textrm{mr}= 3-4\times 10^{-23}\,\textrm{cm}^2$. 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.

An Excited Dark Matter Solution to the MeV Galactic Center Excesses

TL;DR

The paper tackles the Galactic Center puzzles of the 511 keV line, a ~2–3 MeV COMPTEL excess, and anomalous CMZ ionization by proposing a minimal Excited Dark Matter (XDM) model. In this framework, DM inelastically upscatters to a nearly degenerate state, which decays to χ e^+ e^− and injects few-MeV positrons, yielding a bulge-dominated morphology due to a velocity-threshold upscattering . Using a full cosmic-ray propagation treatment with DRAGON2, the authors show that TeV and MeV with cm^2 can reproduce the INTEGRAL/SPI 511 keV morphology, while simultaneously producing a COMPTEL IfA spectrum (via in-flight annihilation and internal bremsstrahlung) consistent with observations, and contributing a radially flat CMZ ionization rate s^{-1}. The scenario remains compatible with unitarity and thermal freeze-out and predicts testable signatures for upcoming MeV-band missions and laboratory searches for a ~10–20 MeV mediator. Overall, the work presents the first unified XDM-based explanation connecting line emission, continuum excess, and CMZ chemistry in the Galactic Center.

Abstract

Recent COMPTEL data analysis reveals a 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 is inelastically upscattered into an excited state , followed by de-excitation producing 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 1.5 TeV DM particle with mass-splitting 4 MeV for an inelastic geometric scattering cross section of . 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.

Paper Structure

This paper contains 17 sections, 12 equations, 8 figures.

Table of Contents

  1. Introduction
  2. Excited Dark Matter Framework
  3. Overview and Physical Picture
  4. Kinematics and Excitation Threshold
  5. Positron Injection Rate
  6. Velocity Dispersion and Escape Velocity
  7. 511 keV Morphology and Positron Propagation
  8. In-Flight Annihilation Excess
  9. Ionization of the Central Molecular Zone
  10. Results
  11. Fit to the 511 keV Morphology
  12. Propagation Uncertainties
  13. In-Flight Annihilation Spectrum
  14. Ionization of the Central Molecular Zone
  15. Discussion and Conclusions
  16. ...and 2 more sections

Figures (8)

  • Figure 1: DM velocity profiles as a function of radial distance from the GC for the velocity dispersion $\sigma_v$ (blue) and escape velocity $v_{\mathrm{esc}}$ (red) for NFW (solid) and cNFW (dashed) DM density distributions. The cNFW profile has inner slope $\gamma=1.2$.
  • Figure 2: 511 keV longitude profiles (left panels) and latitude profiles (right panels) for XDM models compared to INTEGRAL/SPI data (red points). Top: fixed $m_\chi = 1.5$ TeV and varying mass splittings $\Delta m$. Bottom: fixed $\Delta m = 4$ MeV and varying $m_\chi$.
  • Figure 3: 511 keV longitude (left) and latitude (right) profiles for XDM with $m_\chi = 1.5$ TeV and $\Delta m = 4$ MeV, compared to INTEGRAL/SPI data (red points), for three propagation setups: "Default" (solid), "Fast" (dashed) and "Slow" (dash-dotted), following DelaTorreLuque:2023cef.
  • Figure 4: Top-left: COMPTEL IfA spectrum in the bulge region (red points) compared to XDM predictions for multiple $m_\chi$ with $\Delta m = 4$ MeV and $\sigma_{\textrm{mr}}$ fixed by the 511 keV fit. Top-right: Same, for fixed $m_\chi = 1.5$ TeV and varying $\Delta m$. Bottom: Decomposition of the XDM signal for $m_\chi = 1.5$ TeV and $\Delta m = 4$ MeV into internal bremsstrahlung (IB, blue) and in-flight annihilation (IfA, green). The sum (black) is compared to COMPTEL data.
  • Figure 5: Radial profile of the H$_2$ ionization rate in the CMZ for XDM models as a function of cylindrical distance $R$ from the GC. Left: fixed $m_\chi = 1.5$ TeV and varying $\Delta m$. Right: fixed $\Delta m = 4$ MeV and varying $m_\chi$. All cases use NFW and the best-fit $\sigma_{\textrm{mr}}$ from the 511 keV morphology.
  • ...and 3 more figures