Spectral diversity in collisional neutrino-flavor conversion: Flavor equipartition or swap

Quantum kinetics of neutrinos are known to potentially change the classical neutrino radiation field in high-energy astrophysical sources such as core-collapse supernovae and binary neutron-star mergers. However, the mixing phenomena still have open issues in the nonlinear dynamics and the asymptotic states, particularly for recently discovered collision-induced flavor conversion. In this paper, we investigate linear and nonlinear dynamics of collisional neutrino-flavor conversion (CFC) with multienergy neutrino gases through numerical simulations, demonstrating that the asymptotic states dramatically change depending on unstable modes dominating the system. In one unstable mode, high-energy neutrinos reach a flavor equipartition, but low-energy neutrinos return back to almost their initial states. In contrast, in the other one, rather low-energy neutrinos achieve a full flavor swap, but high-energy neutrinos undergo less flavor conversion. We clarify the distinct spectral behaviors in two different ways based on stability analysis and flavor pendulum. Our result suggests that CFC with flavor swap can become crucial at deeper radii with low electron fraction and requires more detailed theoretical modeling of neutrino quantum kinetics.