Milky Way White Dwarfs as Inelastic Dark Matter Detectors

Abstract

We show that white dwarfs in the Galactic Center are a sensitive probe of inelastic dark matter (IDM) annihilation. IDM particles traversing a white dwarf may become gravitationally captured without fully settling in its interior if their energy drops below the kinematic threshold for inelastic scattering. In this regime, a fraction of the incoming particles remains bound on long-lived orbits that extend beyond the white dwarf's volume, enabling annihilation to partially proceed outside of the object. Some of the annihilation products subsequently escape, rather than being absorbed by the stellar material, producing a potentially detectable signal. We estimate the collective gamma-ray flux from the external annihilation sourced by these systems, as a function of the IDM parameters, and compare it to H.E.S.S. observations within the central degree of the galaxy. From this comparison, we derive tentative limits on the IDM scattering cross-section at the TeV mass scale, under largely conservative assumptions for the IDM density in this region. As a concrete application, we map these results into specific parameter constraints for dark photon-mediated and higgsino models with mass splittings at the MeV scale.