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A New Spin

X rays shed light on black holes

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10:03am, December 31, 2006
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For an object that wields such a powerful and complex influence, a black hole is amazingly simple. Although its gravity is strong enough to trap light and slow time until each second lasts an eternity, the basic astrophysical properties of a black hole can be described by just two properties: mass and spin rate.

Over the past 15 years, astronomers have made great progress in measuring the mass of big black holes, which may lurk at the center of every large galaxy. Astronomers have found that these black holes weigh millions to billions times the mass of the sun.

Spin rate has been more difficult to measure but has now yielded to astronomers' probes. Measurements of spin provide a critical new test of Einstein's general theory of relativity, which predicts that spinning black holes drag space-time along with them. The new observations lend support to the theory.

Spin measurements provide a new view of the Alice-in-Wonderland world of twisted space and warped time near a black hole. That's because the rotation permits material surrounding the heavyweight to safely orbit at closer distances, instead of getting sucked in, as it would if the hole were stationary.

"Spin imparts unique signatures on the structure of space-time around black holes," note theorists Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass and Eliot Quataert of the University of California, Berkeley in a recent article.

Astronomers don't yet have a spacecraft to journey near a black hole and relay back information about its rotation. But a flotilla of Earth-orbiting X-ray telescopes is recording the fireworks emanating from the region immediately surrounding several supermassive black holes, which lie millions to billions of light-years away.

The new clocking of spin comes from several hundred hours of observations from a recently launched Japanese X-ray telescope called Suzaku. Studies with Suzaku are also fleshing out two proposed sets of structures surrounding black holes: a close-in disk and an outer region of gas.

After years of predictions, "we're just at last starting to see the effects of strong gravity around a black hole," says Andrew Fabian of the University of Cambridge in England.

Iron findings

The Suzaku discoveries rely on the detection of a wavelength of X-ray radiation emitted by hot iron atoms in the gas surrounding a black hole. In the absence of gravity and movement, radiation from the iron atom would produce a single spike on the craft's spectrograph, corresponding to an energy of 6.4 kiloelectron volts.

The mind-bending physics of black holes, however, makes the spectrum recorded by Suzaku much broader, more like a mountain range than a sharp peak. A wide, asymmetric feature, known as the broad iron K line for its appearance on the spectrograph, contains a wealth of information about the interplay between black holes and their surroundings, says Fabian.

Just as a police officer uses a radar gun to record the speeds of passing cars, Fabian and his colleagues employ the spectrograph reading to clock the velocity of material whirling around black holes. From that speed, they deduce how close to the hole that material resides.

Consider that distant Pluto orbits the sun much more slowly than innermost-planet Mercury does. Similarly, distant material orbits a black hole more sedately than nearby material does. The broad X-ray spectrum recorded by Suzaku implies fast-moving, and therefore close-in, material.

Fabian's team has also used the new X-ray data to demonstrate that the gravity of the black holes has slowed time, stretching the radiation to longer wavelengths.

The observations, which the researchers describe in an upcoming Publications of the Astronomical Society of Japan, are providing new evidence that the pattern of X-ray emission from iron atoms around a black hole originates from two different processes. The first is the rotation of the disk, while the other is the strong gravitational pull of the black hole.

Most material drawn toward a black hole doesn't dive directly in but forms a swirling disk, called an accretion disk, around the hole. That disk continuously feeds the voracious monster.

When a disk is viewed edge-on from Earth, material heading toward the back side of the black hole recedes from our planet, while material spinning toward the front approaches us. Like the changing pitch of a wailing ambulance as it moves past an observer, X rays emitted by the approaching material are shifted to shorter, or bluer, wavelengths, while those from the receding material get shifted to longer, or redder, wavelengths.

These blue and red shifts broaden the spectrum of X rays emitted from the iron atoms in the accretion disk. The greater the speed of the disk, the more spread out the spectrum becomes. Specifically, the width indicates the speed of the innermost part of the disk—the gas that's moving around the black hole the fastest.

"The Suzaku observations are beautifully confirming the models [of spin] that we have been applying" from earlier data, says theorist Chris Reynolds of the University of Maryland at College Park.

Fabian and his colleagues recently trained Suzaku on a galaxy called MCG-6-30-15. The width of the X-ray spectrum indicates that the galaxy's central black hole is whirling around at 90 percent of its maximum possible rate of rotation. Had the black hole been rotating any slower, it couldn't have whipped nearby space-time into quite as strong a tornado, and the inner part of the accretion disk couldn't have extended quite so close to the event horizon—the boundary between the black hole's maw and the outside world. Instead, the material would have plunged in.

With the extensive data that they collected, Fabian and his colleagues, including Giovanni Miniutti of the University of Cambridge, could distinguish a second factor that contributes to the broadness of the X-ray radiation emitted by the iron atoms. It's a shift distinctive from the red and blue shifts attributed to material speeding toward and away from Earth. Known as gravitational redshift, the effect is caused by the black hole's powerful gravity, which slows time and causes light waves to lose energy. The loss in energy causes the light waves to have longer wavelengths. Previous studies hadn't demonstrated this redshift as clearly.

In the case of MCG-6-30-15, the gravitational redshift is so extreme, says Fabian, that most of the emission must be coming from the gas and dust extremely close to the black hole. The team calculates that the inner edge of the disk is only twice as far away from the hole's center as is the event horizon. That proximity of the disk is another indicator that the black hole is a whirling dervish, spinning so rapidly that outside material can orbit within a hair's breadth of ultimate doom.

From previous X-ray measurements, astronomers hadn't been certain that they were seeing the effects of strong gravity from a black hole, notes James Reeves of NASA's Goddard Space Flight Center in Greenbelt, Md., and Johns Hopkins University in Baltimore. But the Suzaku measurement "leaves little doubt," he adds.

Scientists now have sufficient data to measure black hole features other than spin. For example, they've calculated the orientation of an accretion disk. In a galaxy called MCG-5-23-16, Reeves' team determined that the accretion disk is angled at 45° to the black hole's axis of rotation.

The angle of the accretion disk indicates how much gas and dust an Earth-orbiting telescope must look through as it views a black hole. With this information, scientists can more accurately determine a black hole's gravity, energy, and other features.

Outer limits

The new observations are also solving a puzzle about the material surrounding black holes. Although the accretion disks emit X rays, computer simulations have indicated that the disks around black holes are, in astronomical terms, cool and thin. The gas is warm enough to generate visible and ultraviolet light but not the billion-degree temperatures required to generate X rays.

Theorists propose that outside the accretion disk lies a cloud of highly ionized gas—the corona. Just as the sun's corona, or outer atmosphere, is hotter than the surface of the sun, the black hole's corona would be hotter than the closer-in accretion disk. Some of the radiation produced by the corona would shine on the disk, exciting the gas within it. That extra energy would trigger iron atoms in the disk to fluoresce and generate the X rays that produce the broad iron K line.

The corona seems responsible not just for the X rays but also for wild variations in X-ray intensity that Fabian and his collaborators have observed in the vicinity of some black holes. The team proposes that the changing intensity isn't the result of variations in the amount of light emitted by the accretion disk.

"The corona is dancing around," says Reynolds, and when the corona gets closer to the hole, more of its light is trapped by the hole, less falls on the accretion disk, and the X rays appear dimmer. When the corona moves farther from the hole, more of its light travels freely into space, and the corona appears brighter.

The great intensity of the X-ray emissions measured can't be completely accounted for by the gravitational energy of supermassive black holes. The X-ray power could be a sign that magnetic fields extend outward from the hole and thread around the spinning accretion disk. The fields could then act to extract rotational energy from the hole and transfer it outward.

Some of this energy might be funneled into the giant jets of high-speed gas often observed to be blasting out from the neighborhoods of supermassive black holes. If correct, this magnetic model would solve a long-standing puzzle of how these jets are powered, Reynolds notes.

Suzaku observations by Reeves' team are also revealing that the broad iron K line "is quite common," emanating from many massive galaxies, says Fabian. The detailed observations that the craft has made in a handful of cases may therefore apply to supermassive black holes across the universe.

"All these effects have been very elusive," says Fabian, adding that the Suzaku observations "take us into a new regime."


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