X-ray flashes illuminate general relativity.

Observations of rapid oscillations in the intensity of X rays emitted by gas striking a neutron star's surface provide a novel test of Einstein's general theory of relativity.

A typical neutron star--the extremely dense cinder left behind after a violent supernova explosion--consists of matter roughly equivalent to the sun's mass compressed into a ball only about 15 kilometers in diameter. General relativity predicts that such a concentration of mass would curve space-time in the star's vicinity so much that there would be observable effects in the behavior of gas plunging to the neutron star's surface.

Now, William Zhang of NASA's Goddard Space Flight Center in Greenbelt, Md., and his coworkers have uncovered evidence of such behavior in data from the Rossi X-ray Timing Explorer satellite. Zhang reported the findings this week at a meeting of the American Physical Society in Columbus, Ohio.

This is "the first evidence of a unique effect of general relativity in regions of strongly curved space-time--an effect that has never been observed before," says Frederick K. Lamb of the University of Illinois at Urbana-Champaign.

Zhang and his team studied a binary system in which the two stars are so close together that the neutron star's strong gravitational field pulls gas off the surface of its companion, an ordinary star. That gas circulates around the neutron star to form an accretion disk. Swirling inward, streams of gas eventually fall onto the neutron star's surface, generating temperatures as high as 100 million kelvins and emitting X rays.

Soon after the Rossi satellite's launch in 1995, researchers discovered that the intensity of such X-ray emissions could rise and fall more than 1,000 times per second. Lamb and other theorists proposed a more detailed picture of disk behavior to account for the rapid oscillations.

In the wide outer ring of the accretion disk, gas travels in nearly circular orbits, slowly spiraling inward. At a certain distance from the neutron star's surface, however, radiation from the star becomes so intense that it impedes the flow of the gas. Unable to orbit, gas inside that boundary plunges rapidly to the star's surface, creating a bright spot wherever it hits.

The researchers hypothesize that a clump of gas at the boundary between the accretion cloud's inner and outer regions would deliver extra gas to a particular spot on the neutron star's surface once every orbit. Observers detecting X rays from that spot would see corresponding oscillations in intensity, which reflect the characteristic orbital speed.

For reasons not yet understood, the rate of gas flow within an accretion disk can vary considerably. As the flow rate increases, the boundary between the outer and inner regions gets pushed closer to the neutron star. The frequency of the X-ray brightness oscillations increases to reflect the higher orbital speed at the new boundary.

According to general relativity, however, the extreme curvature of space-time would prohibit stable circular orbits within a certain distance of the neutron star's surface. Calculations by Lamb and his colleagues predict that no matter how high the gas flow rate gets, the observed oscillation frequency eventually levels off.

Made more than a year ago, the prediction triggered a competition among groups observing different stars. Zhang's team is the first to detect such a leveling off in the data from one X-ray source.

Because the maximum attainable frequency depends only on the neutron star's mass and spin rate, researchers now have a new way to determine a star's mass. In this case, the observed maximum frequency indicates a mass 2.3 times that of the sun.

Nuclear physicists had expected that no compact star this massive could resist collapsing into a black hole. If the finding is confirmed, Lamb notes, "it tells us something new about superdense matter in neutron stars."