Experimental Limits on the Radiative Decay of SN 1987A Neutrinos

Data from the Gamma-Ray Spectrometer on the Solar Maximum Mission satellite has been searched for excess $\gamma$-ray emission coincident with the ∼ 10-s neutrino burst from supernova 1987A. The absence of a detectable signal allows us to state that the energy in MeV $\gamma$ rays during the burst is < ${10}^{-10\mathrm{}}$ of that in supernova neutrinos above 9 MeV. Direct interpretation limits the lifetime of any radiatively unstable massive neutrinos to $\frac{{\tau }_{{\nu }_i}}{m_{{\nu }_i}}\ge 6.3\times {10}^{15}\left(B_{\gamma i}{f}_{\gamma i}\right)$ s/eV for $m_{{\nu }_i}<20\mathrm{}$ eV, where $B_{\gamma i}$ and $f_{\gamma i}$ are the branching ratio for $\gamma$ decay of species $i$ and the fraction of decays giving detectable $\gamma$ rays, respectively.