X-Ray Lines from Dark Matter Annihilation at the keV Scale

Area of Science:

• Astrophysics
• Particle Physics
• Cosmology

Background:

• Hints of an unidentified 3.5 keV X-ray line in astrophysical signals from galaxies and galaxy clusters were reported in 2014.
• This observation has spurred interest in keV-scale dark matter models, particularly those involving particle decay producing such a line.

Purpose of the Study:

• To explore the less-investigated possibility of dark matter annihilation into monochromatic photons.
• To introduce and analyze a novel model of fermionic dark matter (χ) with O(keV) mass annihilating to a scalar state (ϕ) that decays into photons.

Main Methods:

• Proposed a new theoretical model for fermionic dark matter annihilation.
• Investigated dark matter production via misalignment and freeze-in mechanisms.
• Constrained the model using astrophysical X-ray data and analyzed its implications for the 3.5 keV line.

Main Results:

• The model predicts a box-shaped photon spectrum, which can resemble a narrow line if dark matter (χ) and the scalar state (ϕ) are nearly mass-degenerate.
• Both misalignment and freeze-in production mechanisms are viable across large parameter spaces.
• The model shows potential to reconcile various 3.5 keV line observations due to velocity-dependent annihilation cross-sections.

Conclusions:

• The proposed dark matter annihilation model offers a compelling explanation for the 3.5 keV X-ray line.
• The model successfully avoids structure formation constraints typically associated with keV-scale dark matter.
• Further astrophysical observations can test this novel dark matter scenario.