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01/13/94

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New signature for black holes predicted

STANFORD --Unique oscillations that occur in the disks of hot gas that should surround many black holes may provide, for the first time, a signature that enables scientists to determine the mass and rotation rate of these exotic cosmological objects.

Predictions of the existence and nature of these potentially observable oscillations will be presented Friday, Jan. 14, by Professor Robert V. Wagoner and Christopher Perez, who was a doctoral student on the research project, at the meeting of the American Astronomical Society in Arlington, Va.

Black holes are thought to be collapsed stars with a gravitational pull so strong that nothing can escape, not even light. Before it is swallowed by the black hole, the matter that comes within its gravitational grasp should first form into a flattened disk in much the same fashion that the planet Saturn has pulled the material in its vicinity into a series of rings.

By modeling the gravitational field of a black hole in three dimensions, Wagoner and Perez determined that such an "accretion disk" should oscillate in a special way that similar disks surrounding stars and other celestial objects do not. According to their analysis, these vibrations are due to unusual relativistic effects that result from the extreme distortions of space and time that occur in the vicinity of a black hole.

Unlike other proposed signatures for black holes, these vibrations should provide both the mass and the rotation rate of the central black hole, according to Wagoner.

For stellar-sized black holes the physicists estimate that these fluctuations should appear as X-rays and vary on a time scale of milliseconds. For supermassive black holes - those weighing in at 100 million times the mass of the sun, which are thought to be the power source for quasars and other highly luminous galactic cores - the disk oscillations should appear in the visible to ultraviolet range and vary daily. The scientists calculate that these pulsations may be strong enough to detect: on the order of a few percent of the objects' total luminosity.

Their work is based on previous investigations by Wagoner and Michael Nowak, who is now with the Canadian Institute for Theoretical Astrophysics at the University of Toronto, that produced a framework for analyzing pulsations of this type.

The phenomenon is similar in many ways to the oscillations that have been seen on the surface of the sun and a few other stars. Such observations have provided valuable information about the interior of these objects and have spawned new fields of study now called "helioseismology" and "astroseismology." As a result, the scientists have dubbed their work "diskoseismology."

The search for these oscillations in visible light from supermassive black holes has already begun at a few observatories. Rudolph Schild of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and David Thomson at the AT&T Bell Laboratories in Murray Hill, N.J., for example, may have detected such a signature during their extensive monitoring of a gravitationally lensed quasar, Wagoner reported.

Experiments on two astronomical satellites being prepared for launch - the X-Ray Timing Explorer and ARGOS - may also have the capability to detect the X-ray oscillations that should be produced by stellar-sized black holes in the Milky Way galaxy, he said.

In the future, Wagoner said, he intends to improve the accuracy of these predictions by treating the complicated physics of accretion disks in more detail.

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