June 9, 2000 Future orbiting X-ray telescopes should confirm whether gigantic black holes in the universe, some of which are thought to weigh as much as several billion stars, are spinning like flywheels, according to a University of Colorado at Boulder astrophysicist.
Giant black holes, most often located in the centers of galaxies, generally are surrounded by rotating disks of gas replenished by a constant infall of stellar material, said University of Colorado at Boulder postdoctoral researcher Christopher Reynolds. Orbiting X-ray telescopes have detected iron emissions in such rotating disks, known as accretion disks, he said.
Reynolds presented a talk on the subject at the AAS 2000 spring meeting held in Rochester, N.Y. June 4 to June 8.
"We see evidence of iron atoms in the X-ray spectrum of material circling the black hole," Reynolds said. "We can see the effect of time going more slowly as the material approaches the event horizon, or edge, of the black hole, just as predicted in Einstein's theory of relativity. But in order to confirm that these giant black holes themselves are spinning, we need data from more sophisticated telescopes."
Matter falling into the accretion disks, which eventually is sucked into black holes, is visible in most of the electromagnetic spectrum, including X-rays, he said. Iron atoms, in particular, become "excited" by infalling material, absorbing X-rays and emitting unique frequencies in the spectrum. Astronomers can see this emission and use it to trace the material falling into the black hole.
Black holes are believed to be regions where gravity becomes infinite -- so strong that nothing entering them, including light, can escape. Most black holes are believed to be caused by the collapse of stellar bodies after their nuclear fuel has been spent.
A black hole the mass of our sun would have a diameter of only about 3 miles across. Such stellar black holes, believed to pepper each of the estimated 50 billion to 100 billion galaxies in the universe, are far smaller than gigantic black holes, believed by astrophysicists to act as "cosmic vacuum cleaners" and usually located in the center of galaxies. They may harbor debris from millions or billions of stars, all crunched together at a single point known as a "singularity."
Launched in 1993, The United States/Japan satellite, ASCA, has been able to measure X-ray frequencies of iron around giant black holes. "With current X-ray telescopes such as ASCA, we are right on the fringe of what the data can tell us right now about the rotation of these giant black holes," said Reynolds, who received his doctorate at Cambridge before coming to CU-Boulder as a postdoctoral researcher at JILA, a joint institute of CU and the National Institute of Standards and Technology.
"But a more sophisticated telescope system could reveal the variability of X-ray emissions in the accretion disk around the black hole by detecting emission lines of 'excited' iron atoms and other elements as they fall into black holes," he said. Powerful spikes in the X-ray brightness of giant black holes appear to be caused by huge flares in the accretion disks, much like those seen on the sun and stars. "A giant flare sends an echo of X-rays across the accretion disc," said Reynolds.
"In computer simulations, such 'flare echoes' spread out across the accretion disc, with some echoes moving away from the black hole and some moving toward the black hole's center."
A new telescope slated for launch by NASA in the next decade is Constellation X. This X-ray observatory will consist of four telescopes that will fly in formation about 1 million miles from Earth. They will point at single giant black holes, much like radio telescope arrays point at single objects and combine their observation data.
"Since black holes are perfectly round with no surface features, it is difficult to see their rotation," said Reynolds. "But Einstein's theory of general relativity says that the spin distorts the space-time continuum near the black hole.
"Using Constellation-X to look at the X-ray, iron emission line echoes will allow us to detect these distortions," he said. "These observations also will test whether Einstein's theories are correct."
For more information on probing giant black holes with X-ray observations, go to the Web site at: http://rocinante.Colorado.EDU/~chris/aas/reverberation.html.
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