Dark Matter Inelastic Up-Scattering with the Interstellar Plasma: An Exciting New Source of X-Ray Lines, including at 3.5 keV

Understand

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.