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X-rays Hint at Space Pirouette

7 November 1997 8:00 pm
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What a drag. The jump in x-ray intensity at 67 CPS may reveal a spinning black hole dragging the space surrounding it.

Tiny wobbles in x-rays from matter being pulled toward neutron stars may be a sign that these spinning stars are dragging huge swaths of space with them. The findings, announced at a press conference yesterday at a meeting of the American Astronomical Society's High Energy Astrophysics Division in Estes Park, Colorado, may be new evidence for "frame dragging"--a phenomenon, some experts say, that might serve as a tool for better studying black holes.

If you spin in your office chair while holding a full cup of coffee, centrifugal force is likely to leave you wet--even if the entire office building and the surrounding city should somehow spin with you and hide any clue to your motion. That's because, according to one of the more baffling concepts from Einstein's theory of relativity, the spin is measured against an abstract "frame of reference" determined by the average positions of all the stars in the universe--not a nearby speck of matter like a city. But if a chunk of matter is dense and massive enough, relativity predicts it can "drag" reference frames right along with it, as if space, like black coffee, were a substance that could be stirred and swirled.

Now that claim may be backed up by new measurements from the 2-year-old Rossi X-ray Timing Explorer, a satellite carrying sensitive detectors that allow it to observe faint, rapidly varying x-ray signals. In one presentation, Luigi Stella of the Astronomical Observatory of Rome presented an analysis of x-rays from matter spinning around several neutron stars. As this matter spirals into a spinning accretion disk and sporadically plunges toward the neutron star, fluctuations in its x-ray glow tip off astronomers to how fast the matter in the accretion disk is moving and even how fast the neutron star is spinning.

But the Rossi's sensitive detectors have also picked up much slower jitters, which got Stella and colleague Mario Vietri of the University of Rome thinking that a neutron star's powerful magnetic field might create the conditions needed to see frame-dragging in the x-ray signal. Like the whirring blades of an egg beater, the magnetic field lines fling matter from the accretion disk out of the disk's plane. There, like a tilted toy top, the material should wobble, or precess, at the same frequency that the reference frame--and therefore the very space in which it exists--is being dragged around the star.

The wobble frequencies measured by the Rossi satellite are broadly consistent with the amount of dragging expected from the neutron stars' spin rates and the distance at which the material is whirling around them. In a second presentation, a team led by Wei Cui of the Massachusetts Institute of Technology reported that Rossi's measurements of low-frequency x-ray oscillations might have also detected frame dragging around far denser objects: black holes.

"It's an intriguing interpretation," says Mitchell Begelman, an astrophysicist at the University of Colorado, Boulder. "It would be very important if it turns out to be the correct one." If so, he says, the whirling of space could be used to measure how fast other black holes are spinning. And that, in turn, could provide clues as to whether spinning black holes are the engines behind such spectacular displays as quasars, plasma jets, and gamma-ray bursts.

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