Neutrino radiative-lifetime limits from the absence of solarγrays

Observational limits on the solar x- and γ-ray flux restrict the possible conversion of the solar neutrino flux into radiation. This yields a limit on the neutrino radiative lifetime of &≳7×${10}^9$ sec/eV. We discuss the dependence of this limit on the energy of the decay photon and on neutrino masses and mixing parameters. The original application of this method by Cowsik erroneously yields a more re- strictive result due to a misrepresentation of observational data. The present work closely parallels a recent discussion by Vogel on the decay of reactor neutrinos, the solar limits, however, being almost ten orders of magnitude more restrictive. We stress that solar x-ray measurements are an important tool to set neutrino radiative-lifetime limits, although more restrictive limits from other astrophysical data exist.