Dark Matter Inelastic Up-Scattering with the Interstellar Plasma: An
  Exciting New Source of X-Ray Lines, including at 3.5 keV
  We explore the phenomenology of a class of models where the dark matter
particle can inelastically up-scatter to a heavier excited state via
off-diagonal dipolar interactions with the interstellar plasma (gas or free
electrons). The heavier particle then rapidly decays back to the dark matter
particle plus a quasi-monochromatic photon. For the process to occur at
appreciable rates, the mass splitting between the heavier state and the dark
matter must be comparable to, or smaller than, the kinetic energy of particles
in the plasma. As a result, the predicted photon line falls in the soft X-ray
range, or, potentially, at arbitrarily lower energies. We explore experimental
constraints from cosmology and particle physics, and present accurate
calculations of the dark matter thermal relic density and of the flux of
monochromatic X-rays from thermal plasma excitation. We find that the model
provides a natural explanation for the observed 3.5 keV line from clusters of
galaxies and from the Galactic center, and is consistent with null detections
of the line from dwarf galaxies. The unique line shape, which will be resolved
by future observations with the Hitomi (formerly Astro-H) satellite, and the
predicted unique morphology and target-temperature dependence will enable easy
discrimination of this class of models versus other scenarios for the
generation of the 3.5 keV line or of any other unidentified line across the
electromagnetic spectrum.
