Assurance of things not seen

12.18.08 -

By J. William Bell

University of Virginia researchers use TeraGrid resources to simulate the accretion disks that ring black holes and the astrophysical jets they create.

To borrow from a biblical description of faith, accretion disks are an assurance of things not seen. Black holes gobble up light and matter, leaving telescopes nothing to detect. The orbiting gas and plasma that form just beyond the pull of black holes, however, radiate energy in the form of x-rays.


"Black holes became a viable astronomical entity when x-rays [from objects like accretion disks] were detected," says John Hawley, chair of the University of Virginia's astronomy department and a long-time user of NSF-supported supercomputing resources. "They give us evidence, taking things from theory to observation."

Accretion disks around black holes power some of the most energetic phenomena observed in the universe, such as certain binary star systems, powerful quasars, and the enormous jets of radio wave-emitting gas that emerge from the cores of some galaxies. Their energy is staggering—typically more than 10 times what one would get from the same matter undergoing nuclear fusion. And the observed characteristics of these phenomena can fluctuate wildly and very quickly; the x-rays seen in black hole binary star systems wax and wane appreciably on cycles as short as milliseconds.

In 1991, Hawley and his collaborators used NSF-sponsored supercomputing resources to show that otherwise smoothly orbiting gas in an accretion disk will become highly turbulent in the presence of even relatively weak magnetic fields, which had been ignored in previous simulations and theories.

Gas in accretion disks orbits in a balance between gravitational and rotational forces. The magnetic turbulence dissipates some of the gas' orbital speed, breaking the balance and causing the gas to spiral down into the black hole. That dissipated energy takes the form of heat. Because the gas has been orbiting at nearly the speed of light, it can reach millions of degrees Fahrenheit. Such hot gas produces x-rays, revealing the presence of the black hole system. The magnetic turbulence, meanwhile, accounts for the fluctuations in the x-ray emission.

This discovery of the critical importance of magnetic fields "revolutionized our understanding of accretion disks," astrophysicist Omer Blaes wrote in a 2004 issue of Scientific American. "The situation is rather similar to...when astronomers first realized that the primary energy source for stars was nuclear fusion reactions occurring in the stellar core."

Hawley and many other researchers are still coming to terms with that transformative finding. Magnetic fields are central to the behavior of accretion disks, but the finer points remain largely hidden. "We're at the point where we have the fundamental ingredients to understand these disks, to understand the processes going on," Hawley says.

Today, Hawley, postdoc Kris Beckwith, and Johns Hopkins' Julian Krolik and postdoc Scott Noble are investigating astrophysical jets by running large-scale magnetized accretion disk simulations. These jets are beams of energy that sporadically issue from accretion disks and can fire hundreds, thousands, even millions of light years away from disks orbiting supermassive black holes in the centers of galaxies. By simulating the complete disk, magnetic fields and all, as it orbits the black hole, they have seen how these jets can be created. Their most recent simulations have tested the way in which different magnetic field configurations influence both the accretion disk in general and astrophysical jets in particular. So far the strongest jets are produced by the "dipolar" field configuration which has an overall north and south magnetic pole. This configuration is much like the Earth's, except here the fields lie along the axis of a black hole.

These studies were conducted in large part on the San Diego Supercomputer Center's DataStar and are now being switched to NCSA's Abe and the Texas Advanced Computing Center's Ranger supercomputers. Published in a 2008 issue of Astrophysical Journal, early findings showed that changing the orientation and geometry of a system's magnetic field had little impact on the character of the accretion disk itself. They did, however, show that the strength and longevity of astrophysical jets emanating from the disk are very sensitive to the configuration of the magnetic field.

These sorts of simulations—and future simulations that will take place on petascale supercomputers like NCSA's forthcoming Blue Waters—reveal the behavior behind x-ray fluctuations and other features that astronomers see every time they look skyward. They fill in the gaps between theory and observation.

"The dream ultimately is a model that predicts what you would see with an x-ray telescope," Hawley says. "These are preliminary efforts in that direction."

This research is supported by the National Science Foundation and NASA.

Team members
University of Virginia
Kris Beckwith
John Hawley

Johns Hopkins University
Julian Krolik
Scott Noble