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

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