More Hidden Black Hole Dangers
by Christopher Wanjek
Satellite observations now strongly suggest that black holes spin and pull space along for the ride.
A word of advice for cocky deep-space travelers: Dodging a black hole is now a little trickier than simply steering clear of the X-ray-hot star material pouring into the abyss. It seems that some black holes actually spin, much like whirlpools, dragging with them the fabric of space and all that dare to venture near.
Tod Strohmayer of NASA's Goddard Space Flight Center found convincing evidence for spin in a black hole called GRO J1655-40 by studying the pattern of X-rays coming from a mere 49 kilometers outside the black hole's outer boundary, or event horizon. He presented his findings on April 30 at the American Physical Society meeting.
On one hand, the concept of a spinning black hole seems intuitive. After all, everything else in the universe spins, from massive galaxies and white-hot neutron stars to little blue planets and political entities that want to drill for oil in Honest Abe's eye socket on Mount Rushmore. On the other hand, the entire idea is utterly bizarre. How can an object with no surface spin?
"I have a hard time picturing it myself," says Strohmayer, who observed the black hole with the Rossi X-Ray Timing Explorer satellite. "Space around the black hole is shifting." That is, not only is gas flowing into the black hole, but it rides a moving walkway to that terminal of doom.
Black holes such as GRO J1655-40 form from collapsed stars. When stars at least eight times more massive than our Sun exhaust their fuel supply, they no longer have the energy to support their tremendous bulk. These stars explode as supernovae, blasting their outer envelopes into space. If the core is more than three times the mass of the Sun, it will collapse into a singularity, a single point of infinite density.
Although light cannot escape black holes, astronomers can "see" black holes by virtue of the hot, glowing gas ? often stolen from a neighboring star ? that orbits these objects. From our vantage point, the light seems to flicker. The Rossi Explorer has recorded this flickering (called quasiperiodic oscillations, or QPOs) around many black holes. QPOs are produced by gas very near the innermost stable orbit ? the closest orbit a blob of gas can maintain before falling pell-mell into the black hole. As gas whips around the black hole at near light speed, gravity pulls the gas in one direction, then another, adding to the flickering. The QPO is related to the speed and size of this orbit and the mass of the black hole.
Strohmayer observed two QPOs in GRO J1655-40, a previously detected one at about 300 Hertz and a newly detected one at 450 Hertz. GRO J1655-40 is a well-studied system, and earlier optical observations have established the black hole mass at 7 solar masses. Applying Einstein's general theory of relativity, this mass implies an innermost stable orbit of 60 kilometers if the black hole were not spinning.
A QPO at 450 Hertz, however, implies an innermost stable orbit less than or equal to 49 kilometers. That's just the way the math works; a lower QPO would imply a slower, more distant orbit around the black hole. The only way that gas can maintain a stable orbit that fast and close to a 7-solar-mass black hole is if the black hole is spinning. The faster the black hole spins, the closer and faster material can orbit it. (Coincidentally, the 300- Hertz QPO was consistent with a 60-kilometer orbit, so no one raised an eyebrow.)
This isn't the first time scientists have taken us for a spin with a black hole. The Rossi Explorer captured QPOs before that could be explained as the dragging of space-time around a black hole, called Lense-Thirring precession, which would imply that the black hole is spinning. Astronomers are still hotly debating this interpretation. The Japan-U.S. ASCA X-ray satellite spotted distorted X-rays from quasars that could be explained as matter orbiting close to spinning supermassive black holes ? another precarious link.
Strohmayer's observation provides the strongest evidence yet of black hole spin, says Cole Miller, a University of Maryland theorist. "It's the most exciting black hole result I've seen in years," he says, cautioning, however, that the 450-Hertz QPO is weak. Miller would like to see similar observations in other black hole systems.
Virginia Trimble of the University of California at Irvine adds, "It's virtually certain the result is qualitatively correct, unless general relativity breaks down. It's another small step toward a theory of everything." She says that Strohmayer's result helps elucidate how gravity works at its utmost expression, and how this force, described by general relativity, relates to the other known forces characterized by quantum theory.
In May, Strohmayer found paired QPOs in another black hole, GRS 1915+105. The QPOs are no indication of black hole spin, but they do have theorists turning about. Neutron stars often exhibit paired oscillations, thought to originate from radiation bouncing off their solid surfaces. What would generate paired QPOs in black holes? Expect theorists to take QPO theory for a spin in upcoming months.
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