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A Good Belch Helps Black Holes Feast

21 June 2006 (All day)
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CXC/M. Weiss

Blow out.
Magnetic fields drive a wind away from a black hole, new measurements show, helping the hole to consume matter closer to it.

Any science-fiction fan will tell you that a black hole sucks up anything that wanders too close. But to consume the matter swirling around it, a black hole must, ironically, push some stuff away. Precise measurements of a nearby black hole now show that an outward flowing "wind" carries away matter, and that magnetic fields--and not heat or radiation--drive the wind.

If matter felt only a black hole's gravity, it would simply circle the hole forever, just as Earth orbits the sun incessantly. Matter falls in, of course: A rotating "accretion disk" of the stuff orbits the hole, and its inner edge feeds the beast. But in order for this to happen, matter farther out in the disk must move away from the hole, carrying off some of the inertia, or "angular moment," of the in-falling matter. That can happen in two ways. The faster-orbiting inner parts of the disk can rub against the slower-orbiting outer parts, causing the inner parts to slow down and move in and the outer parts to speed up and move out. Alternatively, a wind could carry angular momentum away from the hole. But researchers have debated the importance of such a wind and what might drive it.

Now, detailed observations show that magnetic fields whip up a wind around a black hole in our galaxy. Using NASA's orbiting Chandra X-ray Observatory, a team of astronomers--led by Jon Miller of the University of Michigan, Ann Arbor, Andrew Fabian of the University of Cambridge in the United Kingdom, and John Raymond of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts--studied the black hole, which is 10,000 light years away and is consuming a companion star. The gas around the black hole absorbs x-rays of predictable wavelengths, producing dips or "absorption lines" in the spectrum of x-rays emitted by the hole. The researchers found that the dips were all shifted to slightly shorter wavelengths--proof the gas was flowing away from the black hole in a 500-kilometer-per-second wind.

Moreover, details of the x-ray absorption reveal that the gas is too highly ionized to feel much of a push from radiation and too cold to fly out of the disk by itself, the researchers report 22 June in Nature. That leaves magnetic fields as the only viable explanation, Miller says.

"This is a tremendous step," says Roger Blandford, an astrophysicist at the Stanford Linear Accelerator Center in Menlo Park, California. "There's been vague evidence for out-flows, but this is far more quantitative and prescriptive." Just how the magnetic fields create the wind remains murky, however, says Steven Balbus, an astrophysicist at the École Normale Supérieure in Paris, France. Large-scale magnetic fields reaching out of the disk could slingshot particles into space, as Blanford has proposed. Alternatively, Balbus says, tangled magnetic fields within the disk could increase friction and kick up the wind as a by-product

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