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

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 neglected in the literature. We show that this naive simplification has limited our understanding of the flavor conversion phenomenology: fast pairwise conversions are strongly affected by the neutrino energy. An earlier onset of flavor conversions and a higher oscillation frequency are found as the vacuum frequency increases (i.e., the neutrino energy decreases), and the oscillation periodicity, otherwise present, is lost. 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. Our findings highlight the limitations intrinsic to widely adopted approximations. In order to gauge their possible impact on the source physics, a more sophisticated modeling of pairwise conversions is necessary.