Gamma Rays and the Decay of Neutrinos from SN1987A

We calculate limits to the properties of massive, unstable neutrinos using data from gamma-ray detectors on the Pioneer Venus Orbiter Satellite; a massive neutrino emitted from SN1987A that decayed in flight and produced gamma rays would be detectable by this instruments. The lack of such a signal allows us to constrain the branching ratio to photons ($\Bg$), mass ($\mnu$), and radiative lifetime ($\tau_\gamma = \tau/\Bg$). For low mass ($m<T\sim8\MeV$) neutrinos decaying $\nu\rightarrow\nu'\gamma$, $\Bg<3\times 10^{-7}$, for $\mt\lesssim 10^6 \keV\sec$, and $\Bg<6\times 10^{-14} \mt/\keV\sec$ for $\mt\gtrsim 10^6 \keV\sec$; limits for high-mass neutrinos are somewhat weaker due to Boltzmann suppression. We also calculate limits for decays that produce gamma rays through the \brem channel, $\nu\rightarrow\nu'e^+e^-\gamma$. In the case that neutrino mass states are nearly degenerate, $\delta m^2/m^2\ll1$, our limits for the mode $\nu\rightarrow\nu'\gamma$ become more stringent by a factor of $\delta m^2/m^2$, because more of the decay photons are shifted into the PVO detector energy window.