Were the supernova's neutrinos pulsed?

Were the supernova's neutrinos pulsed?

Astrophysicists have managed to extract a great deal of information and suggestions from the small burst of neutrinos that came from Supernova 1987A and were recorded by the Kamiokande detector in Japan and the IMB detector in Ohio last February. The latest suggestion, reported in the Aug. 6 NATURE, is that the emission of the neutrinos was periodic.

The new analysis comes from Martin Harwit of Cornell University, Peter L. Biermann of the Max Planck Institute for Radioastronomy in Bonn, West Germany, Hinrich Meyer of the University of Wuppertal, West Germany, and Ira Wasserman of Cornell, who find a possible period of 8.9 milliseconds. "While the statistical significance of this period is marginal in each individual experiment,' these observers remark, "the two sets of data are compatible.' That makes them think they may be on to something.

If the periodicity is real, it would arise from rotation of the core of the exploding star. As the star explodes, its core collapses to a neutron star, and the collapse produces a lot of neutrinos. Theorists had imagined that the neutrinos come out equally in all directions. Periodicity indicates that they come instead from a particular spot or spots--"nozzles' is one word these observers use to describe it. If the core rotated, the beam from such a nozzle would sweep around, giving the effect of pulses as it crossed the line of sight from earth.

As it happens, astrophysicists generally expect the core of a supernova to rotate. They expect it eventually to become a pulsar, a rotating neutron star, that gives off pulsed radio and maybe light signals because of the same kind of beaming effect. The apparent 8.9-milli-second period of the neutrinos would mean that that is the rotation period of the core and of the eventual pulsar. "We also note that P=8.9 ms is a reasonable period for very young pulsars,' say the observers.

A further question is whether neutrinos have a very small rest mass. Physicists have believed that neutrinos have exactly zero rest mass, but new theoretical developments would give them a tiny rest mass. The mass, if it exists, can be estimated from the time of flight between SN 1987A and earth. Periodicity makes it possible to time the flight very accurately, and on the basis of the period they find, these observers calculate that the mass of the neutrino-- if it exists--can be no more than 0.2 electron-volts, a number many times smaller than previous estimates.