Global features of fast neutrino-flavor conversion in binary neutron star merger

Binary neutron star merger (BNSM) offers an environment where fast neutrino-flavor conversion (FFC) can vividly occur, that potentially leads to a considerable change of neutrino radiation field. In this Letter, we investigate global features of FFC by general relativistic quantum kinetic neutrino transport simulations in spatial axisymmetry. Our result suggests that global advection of neutrinos plays a crucial role in FFC dynamics. Although flavor conversions occur ubiquitously in the early phase, they can be active only in a narrow region in the late phase. This region includes an ELN-XLN Zero Surface (EXZS), corresponding to a surface where electron-neutrinos lepton number (ELN) equals to heavy-leptonic one (XLN). The EXZS is not stationary, but dynamically evolve in a time scale of global advection. We also find that neutrinos can undergo a flavor swap when they pass through the EXZS, resulting in qualitatively different neutrino radiation fields between both sides of EXZS. Our result suggests that EXZS is one of the key ingredients to characterize FFC in BNSM.