Neutron Stars Spin Out Gravitational Waves

Newborn neutron stars may be powerful beacons of gravitational waves, the ripples in the fabric of space-time predicted by Einstein's theory of gravity. In the 1 June Physical Review Letters, physicists calculate that just after a giant star has collapsed into a neutron star, the superdense neutron matter vibrates in a way that sheds large amounts of energy and angular momentum into gravitational waves. The process could sap so much energy from the spinning neutron star that it might slow in as little as a year from 1000 rotations a second to a leisurely 100, which may explain why astronomers have never spotted a fast-spinning young neutron star.

Although no one has directly detected gravitational radiation, Einstein's theory predicts that clumps of matter emit gravitational radiation when they shake or move, just as moving charged particles spew electromagnetic radiation. The gravitational radiation emitted by people and cars, or even the Earth, is minute. But for supermassive objects like neutron stars, it could be significant. Because neutron stars form when a star that may be spinning slowly collapses to a tiny fraction of its radius, like the proverbial skater drawing in his arms, they can have very high spin rates. Theorists have known for some time that spinning neutron stars can shed their spin through gravitational radiation, but they thought that the process had only a minor effect. The sloshing that generates the gravity waves, they thought, would be damped by friction in the fluid of neutrons and electrons making up the star.

Last year, though, Nils Andersson of Washington University in St. Louis found that a type of vibration pattern, or mode, in the rotating star grew stronger instead of weaker when gravitational radiation was emitted. Called r-modes, the vibrations are a bit like the currents in oceans, says Lee Lindblom, an astrophysicist at the California Institute of Technology (Caltech). Lindblom and his Caltech colleague Benjamin Owen, with Sharon Morsink, a physicist at the University of Wisconsin, Milwaukee, have now gone on to calculate the exact strength of the interaction between gravitational radiation and the r-modes. They found that the feedback leads to gravitational radiation so powerful that it quickly puts the brakes on a fast-spinning star. "Why you don't see young neutron stars spinning fast has long been a puzzle," says Kip Thorne, a Caltech astrophysicist. "Now there is a compelling explanation why."