Freeze-in and freeze-out of sterile neutrino dark matter

A sterile neutrino with a keV-scale mass is a compelling dark matter candidate. We propose a new production mechanism involving the decay and annihilation of a complex scalar singlet with a Higgs portal coupling which develops a vacuum expectation value. The interactions of the resulting pseudo Nambu-Goldstone boson may thermalise the dark sector. We determine the region of parameter space where dark sector thermalisation is reached and discuss the most relevant cosmological observables. The scenario can be considered as the combination of a freeze-in of the dark sector followed by relativistic freeze-out.