An Excited Dark Matter Solution to the MeV Galactic Center Excesses

Abstract Recent reanalyses of COMPTEL data reveal a ∼2 MeV continuum excess whose spatial distribution closely matches the 511 keV line observed by INTEGRAL/SPI (SPectrometer on INTEGRAL), pointing to a common population of low-energy Galactic positrons that is difficult to explain with known astrophysical sources or standard thermal dark matter (DM). We show that a minimal Excited DM scenario provides a unified explanation of these signals. In this framework, a DM particle χ is inelastically upscattered into an excited state χ * , which de-excites via χ * → χe + e − , injecting few-MeV positrons. These positrons reproduce the observed 511 keV morphology and generate the MeV continuum through in-flight annihilation. Using a full cosmic-ray propagation treatment, we obtain remarkable consistency for m χ ≃ 1.5 TeV with a mass splitting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>m</mml:mi> <mml:mo>=</mml:mo> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msup> </mml:mrow> </mml:msub> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≃</mml:mo> <mml:mn>4</mml:mn> </mml:math> MeV, and an inelastic geometric scattering cross section σ mr ≃ (3–4) × 10 −23 cm 2 . The same positrons yield a substantial, approximately radially flat contribution to the anomalous ionization observed in the Central Molecular Zone.