An Excited Dark Matter Solution to the MeV Galactic Center Excesses

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 ${\chi }^{\ast }$, followed by de-excitation ${\chi }^{\ast }\to \chi e^{+}{e}^{-}$ 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 $\Delta m=m_{{\chi }^{\ast }}-m_{\chi }\simeq$ 4 MeV for an inelastic geometric scattering cross section of ${\sigma }_{\text{mr}}=3-4\times 10^{-23}{\text{cm}}^2$. 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.

Comments: 10 pages plus appendices and references, 8 figures