Sterile neutrino dark matter with dipole interaction

We consider the possibility of the lightest sterile neutrino dark matter which has dipole interaction with heavier sterile neutrinos. The lifetime can be long enough to be a dark matter candidate without violating other constraints and the correct amount of relic abundance can be produced in the early Universe. We find that a sterile neutrino with the mass of around MeV and the dimension-five non-renormalisable dipole interaction suppressed by $Lambda_5 gtrsim 10^{15}$ GeV can be a good candidate of dark matter, while heavier sterile neutrinos with masses of the order of GeV can explain the active neutrino oscillations.