MIT scientists have more evidence that black holes can spin, creating a whirlpool in the fabric of space that pumps energy out of the black hole.
Jon Miller, a doctoral candidate at MIT's Center for Space Research, led the observation and analysis of a black hole in the Milky Way. He discussed the findings Monday at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, N.M.
Although matter and light cannot escape black hole XTE J1650-500, energy manages to vent in the form of rotational energy from the spin. Like energy released from a flywheel, this surge of power, conducted along magnetic field lines, brightens the innermost ring of matter orbiting the black hole before it takes its final plunge into the mysterious void.
The observation was made with the European Space Agency's XMM-Newton X-ray satellite.
One striking aspect of the observation, the scientists say, is that XTE J1650-500--a stellar black hole with the approximate mass of 10 suns located about 26,000 light years from Earth--behaves nearly identically to a 10-million-solar-mass black hole more than 100 million light years away in galaxy MCG-6-30-15, as reported in October 2001. This indicates that black holes have profound similarities in the manner in which they accrete matter, regardless of their mass.
"As strange as it sounds, it is very likely that black holes spin, and this has consequences for the matter orbiting and falling into a black hole," said Miller. "The fabric of space itself can be dragged along by the spinning black hole, so matter zips around the black hole on something analogous to a moving walkway at an airport."
Scientists have long thought that black holes spin, just like stars and galaxies, yet they have been hard-pressed to find evidence. After all, a black hole is a singularity, a point of infinite density surrounded by a border known as an event horizon, from which no light can escape.
When a black hole is spinning, matter, usually in the form of extremely hot plasma glowing predominantly in X-rays, can maintain a stable orbit six times more closely than it could around a non-spinning black hole.
With XMM-Newton, Miller's team observed the spectrum of plasma orbiting around XTE J1650-500. An emission line from iron in the spectrum, which would appear as a spike in a more docile setting, was smeared, pulled towards lower energies. This is called gravitational redshifting, the result of the black hole's extreme gravity tugging on photons (particles of light) and draining their energy as they race away from the region.
Miller said the gravitational redshifting in XTE J1650-500 was particularly pronounced, evidence that the matter was extremely close to the black hole and thus evidence that the black hole was spinning, allowing matter to orbit so closely.
"Photons escaping out of the gravitational well lose some energy no matter what," said Miller. "It's like climbing stairs. But when the black hole is spinning, it's like trying to climb up the 'down' escalator."
Furthermore, the photons, which should appear dim from extreme gravity sucking their energy, were actually brilliant. Apparently the matter orbiting the black hole is getting energized from the black hole itself.
"A spinning black hole possesses a tremendous amount of energy, just like a rapidly spinning fly-wheel," said Christopher Reynolds of the University of Maryland at College Park. "We believe that matter in the immediate vicinity of the black hole can tap into the black hole's spin energy and become highly energized. This process occurs because magnetic fields connect the orbiting gas to the rapidly rotating space-time. It is incredibly exciting that we are now seeing this phenomenon in black holes that have widely different masses and environments. It suggests that energy extraction from spinning black holes may be much more generic than we previously realized."
The scientists also relied on data from NASA's Rossi X-ray Timing Explorer and the Chandra X-ray Observatory. A paper summarizing this result has been accepted for publication as a letter to the May 10 issue of the Astrophysical Journal. In addition to Miller and Reynolds, the authors are Rudy Wijnands and Walter Lewin of MIT; Andrew Fabian of the University of Cambridge; Matthias Ehle of ESA, Michael Freyberg of the Max Planck Institute; Michiel van der Klis of the University of Amsterdam; and Celia Sanchez-Fernandez and Alberto Castro-Tirado of LAEFF-INTA.
A version of this article appeared in MIT Tech Talk on May 1, 2002.