Neutron stars twist Einstein’s theory

Black holes. Curved space-time. Light bent by gravity. Albert Einstein’s general theory of relativity has some pretty bizarre implications. In 1918, Austrian physicists Joseph Lense and Hans Thirring calculated that the theory has even more of a twist: Like an eggbeater, a spinning object twirls the very fabric of space-time around it.

Astronomers may finally have found evidence for this strange effect, known as frame dragging or the Lense-Thirring precession. Reporting in the Sept. 1 Astrophysical Journal Letters, the researchers base their findings on the study of rapid variations in the brightness of X rays emitted by neutron stars, superdense remnants of exploded stars. Neutron stars cram more matter than the sun into a sphere only 20 kilometers in diameter.

Study coauthor Michiel van der Klis of the University of Amsterdam notes that the results are consistent with the twisting of space-time. Many astronomers have come to doubt an earlier finding by another team suggesting frame dragging (SN: 11/15/97, p. 308). Even theorists who are skeptical about the interpretation of the new data say the study provides a novel way to probe extreme gravity and superdense objects.

In their study, van der Klis and Peter G. Jonker of the University of Amsterdam and Mariano Méndez, now at the National University of La Plata in Argentina, used NASA’s Rossi X-ray Timing Explorer to examine X rays emitted from three neutron stars. Launched in 1995, Rossi records rapid variations in X rays from objects such as neutron stars that spin 1,000 times per second.

Many neutron stars have a closely orbiting companion of ordinary density. In a feeding frenzy, the star’s enormous gravity rips blobs of material from the companion, gathering the resulting gas into a swirling disk. Gas at the inner edge of the disk orbits the neutron star at nearly the speed of light. As it spirals inward and crashes onto the star’s surface, the gas heats up and emits a torrent of high-energy radiation, mostly X rays.

In 1996, Rossi’s sensors detected oscillations in X-ray brightness from several neutron stars. Surprisingly, these oscillations occur at only a few select frequencies (SN: 11/14/98, p. 318). Because the frequencies shift slightly from second to second, researchers refer to them as quasiperiodic oscillations (QPOs).

Astronomers believe the highest QPO frequency for a neutron star reflects the rate of rotation at its disk’s inner edge. The origin of a lower-frequency QPO is less certain but may represent the difference in rotation rate between the orbiting gas and the spinning neutron star.

The newly reported QPO frequencies occur as a sideband to the lower-frequency QPO, making them oscillations of oscillations. They could be due to frame dragging, says van der Klis.

If the disk happens to orbit at an angle to the plane in which the neutron star spins, the dragging of space-time will cause the disk to wobble like a top, adds Frederick K. Lamb of the University of Illinois at Urbana-Champaign. This could show up as the extra oscillations his team found, van der Klis says.

Einstein’s general theory of relativity predicts that such an effect occurs because space is not a passive medium but an active participant in gravitation. The geometry of space acts on mass and energy, telling them how to move. Similarly, mass and energy act on space, telling it how to curve.

Lamb maintains that the separation in frequency between each of the new sidebands and its neighboring QPO is too great for it to represent frame dragging. Others are more sanguine about the new result. Dimitrios Psaltis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., told Science News that he and Colin A. Norman of Johns Hopkins University in Baltimore have calculated that the new QPO could be indirectly linked to dragging.

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