New Theory, Technologies Expected To Step Up Detection Of Black Holes

February 24, 1997

Two University of Colorado at Boulder astrophysicists believe a theory they developed with Stanford University researchers may help pinpoint more black hole candidates in distant space when used in concert with a unique NASA satellite.

Michael Nowak and Mitchell Begelman said NASA's Rossi X-ray Timing Explorer satellite launched in late 1995 has been used to identify two viable black hole candidates and may signal the existence of others before the year is out.

Nowak and Begelman, with Stanford astrophysicists Robert Wagoner and Dana Lehr, report on the new theory in the March 10 issue of Astrophysical Journal Letters. The CU-Stanford team used the theory to determine that observations of an object known as GRS 1915+105 made by a team from the Massachusetts Institute of Technology indicate the presence of a black hole.

Black holes are believed to be regions of collapsed stars where gravity is infinite -- so strong that nothing that enters them, including light, can escape, said Nowak, a postdoctoral researcher at JILA. A black holes is thought to contain all the debris remaining from the collapsed star after its nuclear fuel is used up.

Ordinary black holes are relatively small objects, said Begelman, chair of CU-Boulder's astrophysics and planetary sciences department. A black hole with a mass equal that of the sun, for example, would have a diameter of only about three miles. In contrast, the sun has a diameter of about 860,000 miles.

The CU-Stanford team has proposed matter coming into the gravitational grasp of a black hole first flattens into a disk known as an accretion disk, which vibrates differently than similar disks surrounding stars and other celestial bodies. The key characteristic -- a high-frequency vibration in the disk -- may be caused by extreme distortions of space and time believed present in the vicinity of a black hole.

Although researchers formerly believed such flickering signals from the vicinity of suspected black holes were random, the telephone-booth-sized Rossi satellite has shown the vibrations have a "special periodicity," said Nowak.

As gas is attracted to the black hole, it begins to compress and behave like "jello," said Begelman. "We think as gas approaches a black hole, there is a large, coherent 'sloshing' that modulates the oscillation of the disk."

The disk at GRS 1915+105, located some 40,000 light-years distant in the Aquarius Constellation, appears to be vibrating at about 67 Hertz, or cycles per second, said Nowak. This corresponds to a black hole with a mass ranging from 10 to 36 times the mass of the sun, depending on the rate of the black hole's rotation.

Although GRS 1915+105 is more than five times distant than the well-known black hole candidate Cygnus X-1, it appears on occasion to be up to 20 times brighter than Cygnus X-1. "We think this may be a function of GRS 1915+105 gorging itself on matter over a short period of time rather than feeding at a steady rate," said Begelman.

The oscillations in the disks adjacent to black holes trigger violent bursts of x-rays detectable with the Rossi satellite, said Nowak. The periodicity of the x-ray vibrations -- which vary on a time scale of milliseconds -- appears to be determined by both the mass and the rotation rate of the black hole.

The researchers from CU-Boulder and Stanford modeled the gravitational field of a black hole in three dimensions to estimate the unique modes of oscillation expected to occur in the disks of ordinary black holes.

Larger objects known as supermassive black holes -- which may contain a mass equal to 100 million suns -- are thought to lie at the center of most galaxies.

CU-Boulder astronomers are one of the larger user groups of NASA's Rossi satellite, garnering nearly 10 percent of the total observing time this year. JILA is a joint institute of CU and the National Institute of Standards and Technology.
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University of Colorado at Boulder

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