Fast-pairwise collective neutrino oscillations associated with asymmetric neutrino emissions in core-collapse supernova

Abstract: We present a linear stability analysis of the fast-pairwise neutrino flavor conversion based on a result of our latest axisymmetric core-collapse supernova (CCSN) simulation with full Boltzmann neutrino transport. In the CCSN simulation, coherent asymmetric neutrino emissions of electron-type neutrinos ($\nu_{\rm e}$) and their anti-particles ($\bar{\nu}{\rm e}$), in which the asymmetry of $\nu{\rm e}$ and $\bar{\nu}{\rm e}$ is anti-correlated with each other, occur at almost the same time as the onset of aspherical shock expansion. We find that the asymmetric neutrino emissions play a crucial role on occurrences of fast flavor conversions. The linear analysis shows that unstable modes appear in both pre- and post-shock flows; for the latter they appear only in the hemisphere of higher $\bar{\nu}{\rm e}$ emissions (the same hemisphere with stronger shock expansion). We analyze in depth the characteristics of electron-lepton-number (ELN) crossing by closely inspecting the angular distributions of neutrinos in momentum space. The ELN crossing happens in various ways, and the property depends on the radius: in the vicinity of neutron star, $\bar{\nu}{\rm e}$ ($\nu{\rm e}$) dominates over $\nu_{\rm e}$ ($\bar{\nu}_{\rm e}$) in the forward (backward) direction: at the larger radius the ELN crossing occurs in the opposite way. We also find that the non-radial ELN crossing occurs at the boundary between no ELN crossing and the radial one, which is an effect of genuine multi-D transport. Our findings indicate that the collective neutrino oscillation may occur more commonly in CCSNe and suggest that the CCSN community needs to accommodate these oscillations self-consistently in the modelling of CCSNe.