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

Using $\gamma$-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 $\frac{{\tau }_{\nu }}{m_{\nu }}\gtrsim 1.7\times {10}^{15}{B}_{\gamma }$ sec/eV; for masses between 20 and 100 eV, ${\tau }_{\nu }\gtrsim 3.4\times {10}^{16}{B}_{\gamma }{\gamma }_{\mathrm{GRS}}^{-\frac{1}{2}}$ sec; and for masses between 100 eV and a few MeV, ${\tau }_{\nu }{m}_{\nu }\gtrsim 8.4\times {10}^{17}{B}_{\gamma }$ sec eV. Here $B_{\gamma }$ is the branching ratio for the radiative decay mode and ${\gamma }_{\mathrm{GRS}}$ is yet to be determined from the data, but should be of order unity. The axion mass limit is $m_a\lesssim 55{\gamma }_{\mathrm{GRS}}^{\frac{11}{94}}$ eV.