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

Abstract

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.

16 pages, 9 figures; v2, version published in Physical Review D

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• Constraints on Axion-Like Particles from X-ray Observations of NGC1275
• Signatures of Pseudo-Dirac Dark Matter at High-Intensity Neutrino Experiments
• Finite-size dark matter and its effect on small-scale structure
• The ups and downs of inelastic dark matter: Electron recoils from terrestrial upscattering
• Polarization of photons emitted by decaying dark matter
• Consistency of Hitomi, XMM-Newton and Chandra 3.5 keV data from Perseus
• Leptoquarks and Real Singlets: A Richer Scalar Sector Behind the Origin of Dark Matter
• Probing Non-Minimal Dark Sectors via the 21 cm Line at Cosmic Dawn
• Detecting Fluorescent Dark Matter with X-ray lasers