Dispelling a myth on dense neutrino media: fast pairwise conversions depend on energy

Abstract: Pairwise conversions of neutrinos have complicated the challenging goal of quantifying the relevance of neutrino microphysics in compact astrophysical objects, limiting the ability to perform numerical simulations and encouraging the employment of semi-analytical tools, such as the linear stability analysis. Given the high neutrino density, the dependence of pairwise conversions on the neutrino energy has been deemed to play a negligible role. We show that fast pairwise conversions are affected by the neutrino energy, especially in the non-linear regime. An earlier onset of flavor conversions and a higher oscillation frequency are found as the vacuum frequency increases (i.e., the neutrino energy decreases); the oscillation periodicity, otherwise present, is gradually destroyed as the vacuum frequency increases. Such effects are, however, not further exacerbated by the inclusion of spectral energy distributions for neutrinos, contrary to what was found for "slow" neutrino-neutrino conversions. In addition, our findings highlight the dependence of fast pairwise conversions on the neutrino mass ordering for realistic values of the neutrino vacuum frequency. Our findings highlight the limitations intrinsic to widely adopted approximations and extrapolations based on the linear regime of fast conversions. In order to gauge their possible impact on the source physics, a more sophisticated modeling of pairwise conversions is necessary.