Induced resonance makes light sterile neutrino Dark Matter cool
  We describe two new generation mechanisms for Dark Matter composed of sterile
neutrinos with ${\cal O}(1)$ keV mass. The model contains a light scalar field
which coherently oscillates in the early Universe and modulates the Majorana
mass of the sterile neutrino. In a region of model parameter space, the
oscillations between active and sterile neutrinos are resonantly enhanced. This
mechanism allows us to produce sterile neutrino DM with small mixing angle with
active neutrinos, thus evading the X-ray constraints. At the same time the
spectrum of produced DM is much cooler, than in the case of ordinary
oscillations in plasma, opening a window of lower mass DM, which is otherwise
forbidden by structure formation considerations. In other regions of the model
parameter space, where the resonance does not appear, another mechanism can
operate: large field suppresses the active-sterile oscillations, but instead
sterile neutrinos are produced by the oscillating scalar field when the
effective fermion mass crosses zero. In this case DM component is cold, and
even 1 keV neutrino is consistent with the cosmic structure formation.
