The telltale dimming of x-rays emanating from a microquasar about 10,000 light years from Earth is the first direct evidence of the rotation of a black hole system. The object in question, GRO J1655-40, consists of a black hole devouring a nearby normal-star companion.
Matter from the normal star collects on an accretion disk orbiting the black hole before taking the final plunge through the event horizon. This jumping-off platform is so hot that matter there glows at x-ray wavelengths.
Seeing this glow and measuring how the glow changes over short time intervals requires the use of a special telescope -- the Rossi X Ray Timing Explorer (RXTE), which takes snapshots at a rate of 1,000 per second.
A common type of x-ray modulation seen in x-ray binary systems, called a quasi-periodic oscillation (QPO), is thought to occur because the hottest x-ray-emitting part of the disk, in its swift orbit around the black hole, is periodically occluded by the black hole itself.
The gravitational fields at work are enormous, since the inner edge of the accretion disk is only tens of kilometers or so from a black hole of about 7 solar masses.
The specific orbital radius can be deduced from the laws of general relativity, which predict a fixed "innermost stable orbit" for matter circling a black hole. In this case, the predicted orbit is about 64 km.
Many theorists believe, however, that a black hole that spins would have a much smaller event horizon, which would permit orbiting matter to attain a much tighter innermost stable position and a correspondingly faster orbital rate.
At last week's APS meeting in Washington, DC, Tod Strohmayer of the Goddard Space Flight Center reported a previously undiscovered QPO pattern in x rays from GRO J16550-40.
The frequency of this QPO, 450 Hz, is the highest ever seen for x rays coming from a black hole system, implying an orbital radius of only 49 km -- a value consistent, Strohmayer says, with a spinning black hole.
A video simulating the spinning black hole system is available at this URL.
(Editor's Note: This story is adapted from PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 538 May 7, 2001 by Phillip F. Schewe, Ben Stein and James Riordon.)
08-May-2001
