Huge pulsars boosted by tiny neutrinos?

Born in the hearts of exploding supernovas, pulsars have confused astronomers since their discovery 30 years ago. These rapidly rotating, radiowave-emitting neutron stars often travel outside the galactic plane, where most stars congregate, and at greater speeds than their celestial brethren.

Theorists have argued that pulsars rocket out of a supernova at the moment of their creation (SN: 6/8/96, p. 358). How this happens has been a mystery. Now, two physicists suggest that an ejection of neutrinos, particles less massive than electrons, boots pulsars out of the galaxy.

When a star goes supernova, a secondary implosion of material takes place, squeezing stellar protons and electrons into neutrons, leaving behind a neutron star. "At its birth, we get a one-time kick," explains Gino Segre of the University of Pennsylvania in Philadelphia. "And the reaction produces neutrinos-very tiny escaping particles." If the neutrinos escaped in all directions, their pulsar would stay in place. However, if the paths of only 1 percent favored a single direction, that asymmetry could accelerate the pulsar to a speed of 450 kilometers per second, according to a report by Segre and Alexander Kusenko in the Dec. 9 Physical Review Letters.

The physicists posit that as neutrinos flee from a newly created neutron star, their path is bent by the magnetic field of the nascent pulsar. Furthermore, transformations of the neutrinos from one type into another could create additional asymmetries (SN: 5/18/96, p. 319). Too small to be held by the star's gravity, neutrinos headed in any given direction would accelerate the star as if a rocket were strapped to its side.

"We're on the border between physics and astronomy," says Segre. "Astronomers might says this is a cute but eccentric idea. But in physics, cute ideas sometimes turn out to really work."