  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.
