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

Binary neutron star merger (BNSM) offers an environment where fast neutrino-flavor conversion (FFC) can vividly occur, which potentially leads to a considerable change of the neutrino radiation field. In this paper, 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 remain 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 the heavy-leptonic one (XLN). The EXZS is not stationary but dynamically evolves in a timescale 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.