For all their mind-boggling effects on geometry, warping the very fabric of space and time, black holes are governed by two just properties–mass and spin.
Although these superdense objects don’t emit light, their mass can be measured with relative ease if they are partnered with a visible star. Astronomers simply record how much of a tug a black hole exerts on its companion. Determining whether a black hole rotates has proven much more of a challenge. Astronomers reported last week strong evidence that black holes spin like whirling dervishes, dragging space-time along with them.
With the Rossi X-ray Timing Explorer satellite, Tod E. Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues examined the X-ray flashes emitted by stellar matter plunging into a nearby black hole. Known as GRO J1655-40, this system lies 10,000 light-years from Earth and consists of an ordinary star orbiting a black hole about 6.3 times as massive as the sun.
A blob of gas circling a stationary black hole this massive can’t maintain itself in an orbit with a radius smaller than 64 kilometers. If the orbiting material ventures any nearer, it’s doomed to fall into the black hole, never to be seen again.
From the region around GRO J1655-40, the satellite detected X rays that flicker 300 times per second. The observed rapidity is just what astronomers would expect from a blob of hot gas orbiting 64 km from the black hole. The satellite, however, also recorded an even faster flickering–an X-ray signal winking on and off 450 times per second.
A radiating blob of gas orbiting a black hole is like a lighthouse beacon sweeping past Earth hundreds of times per second, suggests Strohmayer. The closer the blob gets to the black hole, the faster it orbits.
The most rapid oscillation detected by Rossi can best be explained by blobs of gas that are orbiting 15 km nearer to the hole than indicated by the slower flickering, he says. The material could maintain itself at this close distance only if the black hole spins, Strohmayer asserts. He explains that a spinning black hole alters space-time in such a way that matter can have a stable orbit at a closer distance than it could around a nonrotating black hole.
Strohmayer reported the findings at a meeting of the American Physical Society in Washington, D.C., and will present further details in an upcoming Astrophysical Journal Letters.
“This is the best evidence by far that black holes spin,” says M. Coleman Miller of the University of Maryland in College Park.
That’s not to say the finding comes as a surprise. A stellar black hole forms when a star at least 10 times the sun’s mass collapses to an infinitely dense cinder. Conservation of angular momentum dictates that if the star was rotating, the resulting black hole should rotate even faster.
Still, with so few clues available to tease out the behavior of black holes, demonstrating the rotation “is big news,” Miller says.
