At the American Physical Society Spring Meeting in Washington, D.C. on Monday, Tod Strohmayer of NASA's Goddard Space Flight Center presented concrete evidence that some black holes actually spin as they suck in their surroundings. "Almost every kind of object in space spins, such as planets, stars and galaxies," he says. "With black holes, it's much harder to directly see that they are spinning, because they don't have a solid surface that you can watch spin around." But he proved his case nevertheless using data from NASA's Rossi X-ray Timing Explorer Satellite.

Strohmayer looked at GRO J1655-40, a microquasar black hole about 10,000 light years from earth. Jets of high-speed particles shoot straight up and down from this type of black hole (see image). And among the emanations, Strohmayer noticed two distinct patterns of flickering x-rays known as quasiperiodic oscillations, or QPOsone previously detected QPO at 300 Hertz (Hz) and a new one at 450 Hz. Scientists had only ever detected this sort of radiation around spinning neutron stars.

The QPO at 450 Hz presented a puzzle. From it, Strohmayer calculated that GRO J1655-40's innermost stable orbitthe closest anything can circle the black hole without falling in-should be 49 kilometers or less. And yet based on the black hole's mass, estimated to be seven times greater than that of the sun, the innermost stable orbit should be 64 kilometers. The only way to resolve the contradiction, Strohmayer says, is if GRO J1655-40 spins: "A spinning black hole modifies the fabric of spacetime near it. The spinning allows matter to orbit at a closer distance than if it were not spinning, and the closer matter can get the faster it can orbit. For GRO J1655-40 we can now say that the only way for it to produce the 450 Hz QPO is if it is spinning."