Return of the X-rays: A New Hope for Fermionic Dark Matter at the keV Scale

A long time ago (in 2014), in galaxies and galaxy clusters far, far away, several groups have reported hints for a yet unidentified line in astrophysical X-ray signals at an energy of 3.5\,keV. While it is not unlikely that this line is simply a reflection of imperfectly modeled atomic transitions, it has renewed the community's interest in models of keV-scale dark matter, whose decay would lead to such a line. The alternative possibility of dark matter annihilation into monochromatic photons is far less explored, a lapse that we strive to amend in this paper. More precisely, we introduce a novel model of fermionic dark matter $\chi$ with $\mathcal{O}(\text{keV})$ mass, annihilating to a scalar state $\phi$ which in turn decays to photons, for instance via loops of heavy vector-like quarks. The resulting photon spectrum is box-shaped, but if $\chi$ and $\phi$ are nearly degenerate in mass, it can also resemble a narrow line. We discuss dark matter production via two different mechanisms -- misalignment and freeze-in -- which both turn out to be viable in vast regions of parameter space. We constrain the model using astrophysical X-ray data, and we demonstrate that, thanks to the velocity-dependence of the annihilation cross section, it has the potential to reconcile the various observations of the 3.5\,keV line. We finally address the $\phi$-mediated force between dark matter particles and its possible impact on structure formation.
Comment: 8 pages, 3 figures. v2: reference added, version matched to journal