Neutron star kick driven by asymmetric fast-neutrino flavor conversion

Multi-dimensional nature of core-collapse supernova (CCSN) leads to asymmetric matter ejection and neutrino emission, that potentially accounts for the origin of neutron star (NS) kick. Asymmetric neutrino radiation fields are, in general, accompanied by large-scale inhomogeneous fluid distributions, in particular for electron-fraction ($Y_e$) distributions. Recently, it has also been revealed that lower $Y_e$ environments in proto-neutron star envelope can offer preferable conditions for collective neutrino oscillations. In this paper, we show that a dipole asymmetry of fast neutrino-flavor conversion (FFC), one of the collective neutrino oscillation modes, can power a NS kick, and that it would generate a characteristic correlation between asymmetric distributions of heavy elements in the ejecta and the direction of NS kick. We strengthen our argument for the FFC-driven NS kick mechanism by performing axisymmetric neutrino transport simulations with full Boltzmann neutrino transport. We show that this mechanism can generate sufficient linear momentum of neutrinos to account for typical proper motions of NS. Although more detailed studies are necessary, the present study opens a new channel to give a natal NS kick.