New Scientist: Do pulsars get their kicks from neutrinos?

Jeff Kanipe

New Scientist, Sat, 07 Dec 1996 00:03:00

ONE of the biggest mysteries about pulsars is not why they pulse, but what force initially propelled them through space at speeds that are great enough to eject them from the disk of the Galaxy.

Pulsars are neutron stars, which are themselves the collapsed cores of supernovae. According to a new theory, the simplest explanation for their speed is a subatomic drama in the collapsing core that culminates in a violent, and slightly uneven, blast of neutrinos.

Most pulsars are between 20 and 30 kilometres in diameter, with intense magnetic fields and rapid spins. In extreme cases, pulsars can spin more than 100 times a second. From Earth, they appear to pulsate because the beams of radio waves they emit from their poles sweep past the Earth with every revolution.

Since the discovery of the first pulsar in 1967, astronomers have struggled to explain why they travel so fast. Although most researchers agree that the high velocities of pulsars must be related to their origin in supernova explosions, the exact cause of the pulsar's "punch" has remained a mystery.

In a paper to be published in the 9 December issue of Physical Review Letters, Alexander Kusenko and Gino Segre of the University of Pennsylvania argue that the velocity kick that propels pulsars through space may be the result of a flood of neutrinos rushing out from the collapsing core, but in a nonuniform way, like the air from a balloon.

When a massive star explodes, it can temporarily outshine a galaxy of several hundred billion suns. But even more staggering is the energy released in the form of neutrinos, which can briefly exceed the luminosity of all stars in the known galaxies. "Most of the energy involved with gravitational contraction of the supernova core is converted into heat," says Kusenko, "and 99 per cent of that energy is carried away by neutrinos." But the neutrino emission may not be uniform-and this may be what causes the kick.

Neutrinos usually pass through normal matter. But the hot interior of a recently-formed neutron star is so dense that neutrinos begin to interact with it. The three types of neutrino-electron, tau and muon-interact with the matter in slightly different ways, with the more massive muon and tau varieties able to escape from deeper within the neutron star.

It is possible, the researchers say, that during this interaction neutrinos may flip from one type to another: an electron-type neutrino may change to a tau type, which could then quickly escape. It turns out that the distance over which these flips occur depends on the direction of the magnetic field in that region of the neutron star, so that on one side, far more tau neutrinos will emerge from its hotter inner regions. "You open the door on one side and let out the hot particles, but you don't open the door on the other side," says Kusenko. "This is quite similar to how a rocket flies."

According to Kusenko and Segre, a difference of just 1 per cent in the total momentum of neutrinos emerging in either direction would result in a kick, or "recoil velocity", that is consistent with the measured average pulsar velocity of 500 kilometres per second.

Jeff Kanipe

From New Scientist magazine, vol 152 issue 2059, 07/12/1996, page 23