WASHINGTON, D.C.--A furiously blinking x-ray source near the center of the Milky Way has given the best evidence to date that black holes spin, astronomers reported here 30 April at a meeting of the American Physical Society. But before anyone can say for certain how fast the hole is spinning, theorists must figure out what makes a black hole blink.
Astronomers were already almost certain that the black hole rotates. That's because it probably formed during the implosion of a heavy star, which are known to rotate. Even if the star was rotating very slowly before it collapsed into a black hole, the collapsing material that ultimately forms the nascent hole must spin ever faster for the same reason whirling figure skaters accelerate as they pull in their arms. But proving that a black hole pirouettes is tricky.
The new evidence comes from a phenomenon observed during two blasts of x-rays, coming from a source called GRO J1655-40. During x-ray flares in 1994 and 1996, a small percentage of the x-ray light, within a narrow frequency range, winked on and off about 300 times per second. Theorists speculate that so-called quasi-periodic oscillation was caused by bright blobs in the black hole's accretion disk, made up of gas that slowly spirals towards the hole. These blobs would shine a beam of x-ray light in our direction like the headlights of passing cars on a circular racetrack. Calculations showed that if the hole wasn't rotating, these blinking blobs in GRO J1655-40 must be on the verge of falling into the hole to reach 300 cycles per second.
But in archival Rossi X-ray Timing Explorer data from the 1996 outburst, Goddard Space Flight Center astronomer Tod Strohmayer noticed winks at 450 Hz. The only explanation for this faster blinking, he says, is that the black hole is spinning, thereby putting a slightly different dimple in the fabric of space-time that allows the disk to get closer, and the blobs to spin faster, without being devoured. If Strohmayer's right, it marks the first definitive detection of a spinning black hole.
That would mean more work for theorists. The frequencies of the two groups of whirling blobs are "strikingly inconsistent" with the predictions of any of several proposed mechanisms for creating such a pair, says astrophysicist Fred Lamb of the University of Illinois, Urbana-Champaign. All the same, Lamb says, "this is a remarkable discovery."