Further evidence for frame dragging.
Albert Einstein's general theory of relativity predicts that a spinning object should drag the fabric of space along with it. As a result, the rotation or revolution of any other nearby body should precess, or wobble. Even the Earth is supposed to exert this "frame-dragging" force on its environs, and the Gravity Probe B spacecraft will try to measure this effect once it is launched in 2000. But Gravity Probe B's gyroscopes will accrue an Einsteinian wobble of only one ten-thousandth of a degree yearly.
By contrast, if something orbits a spinning neutron star or black hole at close range, it might precess a full 360 [degrees] dozens or hundreds of times each second. Although dizzying to contemplate, two research groups claim to see signs of this phenomenon. Both groups are using the Rossi X-ray Timing Explorer to scrutinize signals from binary systems that contain compact objects. Their reports appear in the January 1st Astrophysical Journal Letters.
Luigi Stella (Astronomical Observatory of Rome) and Mario Vietri (University of Rome) focused on X-ray sources that contain neutron stars. Those objects are thought to be surrounded by disks of hot, viscous matter from which copious X-rays emanate. As Stella and Vietri see it, part of the accretion disk in each system is tilted, and that portion precesses because the neutron star is spinning. In a still unspecified fashion, this somehow modulates the X-ray emissions from each of the systems they studied.
Inspired by Stella and Vietri's findings, Wei Cui (MIT) and his colleagues went on to apply similar logic to four black-hole-bearing binaries. They found that two sources stood out: X-rays from GRS 1915+105 in Aquila flicker at a nearly constant rate of 67 hertz, while those from GRO J1655-40 in Scorpius evince a 300-hertz period. Cui's team ascribes the flickering to frame dragging by rapidly rotating black holes.
Intriguingly, an earlier study of GRS 1915 +105 and GRO J1655-40 had already made an indirect case for rapidly spinning black holes in them (S&T: December 1997, page 18). Nevertheless, theorists Mitchell C. Begelman (University of Colorado) and M. Coleman Miller (University of Chicago) find the conclusion tentative at best. Earlier this year Begelman coauthored an alternative explanation for the X-ray fluctuations in GRS 1915+105: drumheadlike oscillations in the accretion disk.
On the other hand, as Begelman and Miller see it, Stella and Vietri's study provides stronger evidence for relativistic precession. There, a trio of neutron-star systems showed variations at three frequencies each. The scientists attribute these frequencies to the neutron star's spin, the orbital period of material along the accretion disk's inner edge, and the frame-dragging effect. In each case, these quantities are consistent with a frame-dragging neutron star.
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|Title Annotation:||an effect of the rotational force of Earth|
|Publication:||Sky & Telescope|
|Date:||Mar 1, 1998|
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