  Nonradiative neutrino decay, which is not satisfactorily constrained,
possibly and significantly changes the detected neutrino signal from galactic
supernova explosions. We focus on the appearance of a sharp peak due to a
neutronization burst in the time profile; this phenomenon would occur if the
original \nu_e, produced at the neutrinosphere and becoming \nu_{2,3} at the
stellar surface, decays into a lighter antineutrino state such as \bar\nu_{1,2}
coupled to \bar\nu_e. This is due to the fact that the signature of the
neutronization burst is common to all numerical simulations. Therefore, the
appearance of a neutronization peak in the \bar\nu_e signal, if it were
detected, would clearly indicate the nonstandard properties of neutrinos. Using
a newly developed formulation that includes flavor conversions inside the
supernova envelope and neutrino decay during propagation in a vacuum, we
calculate the expected neutrino signal at the detectors; the lifetimes of three
modes \tau_{12}, \tau_{13}, and \tau_{23} are taken to be free parameters. We
further introduce simple quantities, which represent a peak sharpness of time
profile and spectral hardness, and investigate the parameter dependence of
these quantities. As the result, it is found that they are quite dependent on
the relevant parameters, but it would be quite difficult to distinguish models
using the signal obtained by the Super-Kamiokande detector; the future
megaton-class detectors would have sufficient sensitivity. We also compare the
neutrino decay model with another mechanism, i.e., resonant spin-flavor
conversion, which also may give the appearance of a neutronization peak, and
conclude that these two independent mechanisms give a different signal and one
can be distinguished from the other.
