X-rays from a distant neutron star have shown that its massive gravity is warping the motions of nearby objects just as Einstein's theory of relativity predicts. Since the early 1940's, physicists have expected that a very dense object, such as a black hole or a neutron star, would bend space so steeply that anything orbiting too close would slide catastrophically inward. X-ray signals, described yesterday at a meeting of the American Physical Society in Columbus, Ohio, now offer the first strong evidence for this smallest stable orbit.
Most astronomers believe that the fastest x-ray flickers of a neutron star originate from streams of material hurtling about the star at the innermost edge of an accretion disk. The point where the material spills into the star moves around as the accretion disk rotates, sending out x-rays like a beacon from a lighthouse. The frequency rises whenever the inner edge of the disk creeps closer to the star, approaching the innermost stable orbit. This may happen when a big chunk of material happens to fall in and blocks some of the neutron star's radiation, which keeps the disk at bay.
Hoping to spot the innermost orbit, astronomers watched a neutron star called 4U 1820-30 with NASA's Rossi X-Ray Timing Explorer satellite over the course of a year. They clocked the frequency of the beacon and watched its brightness, which told how much material was falling in. As expected, when the beacon got brighter, the frequency increased. Then, to their delight, the frequency seemed to hit a ceiling, implying that no smaller orbit was possible. The group measured the limit on four different occasions. "I'm convinced it's not a fluke," says William Zhang, a physicist at NASA Goddard Space Flight Center, in Greenbelt, Maryland.
"These are extremely exciting results," says Massachusetts Institute of Technology physicist Paul Joss. The apparent existence of black holes, he says, is good circumstantial evidence that Einstein's relativity works around dense objects, but this could give "direct evidence that these darn things are working as they should." University of Illinois astrophysicist Frederick Lamb agrees, but cautions that the finding needs to be verified by others. "We need to kick the tires and see that it stands up," he says.