Limits to the Radiative Decays of Neutrinos and Axions fromγ-Ray Observations of SN 1987A

Using γ-ray observations made by the Solar Maximum Mission Gamma Ray Spectrometer in the energy range 4.1-6.4 MeV we limit the possible radiative decays of neutrinos and axions emitted by SN 1987A. For neutrinos less massive than about 20 eV our constraint is τ_νm_ν1.7×10¹⁵B_γ sec/eV; for masses between 20 and 100 eV, τ_ν3.4×10¹⁶B_γγGRS^-1/2 sec; and for masses between 100 eV and a few MeV, τ_νm_ν8.4×10¹⁷B_γ sec eV. Here B_γ is the branching ratio for the radiative decay mode and γGRS is yet to be determined from the data, but should be of order unity. The axion mass limit is mₐ55γGRS11/94 eV.