Magnetic thrust: fields force matter into black holes.

Like a child scared to dive into a pool, cosmic matter needs one last push to plunge into a black hole. New observations confirm that magnetic fields provide this final galactic shove.

The observations come from a system in the Milky Way called GRO J1655-40, which consists of a black hole and a normal star. Gas from the star is pulled toward the black hole, where it forms what's called an accretion disk. Angular momentum--the same property that keeps Earth from diving into the sun--keeps the disk revolving around the black hole instead of falling into it.

Unless something knocked them off course, the gases in the disk would continue to circle the black hole forever. "To get matter in toward the black hole, we have to change the orbits in the disk," says study leader Jon Miller of the University of Michigan in Ann Arbor. Until now, scientists weren't sure whether magnetic fields, radiation pressure, or heat alter the orbit and trigger that fall.

Using NASA's Chandra satellite observatory, Miller's team collected data on X rays emitted from the J1655 system. They found that the X rays came from a wind blowing from the disk, so some force must propel the wind past the black hole's gravitational pull. After simulating the wind on a computer, the researchers conclude that only magnetic fields could create such a force, they report in the June 22 Nature.

The team had ruled out a wind fueled by heat from the core of the disk or by pressure from radiation blasting out of the disk. A heat-driven wind would have been hotter than the one that the researchers observed, and radiation pressure is too weak to drive that wind.

"Magnetic pressure really was the only viable means remaining," Miller says. Such pressure could upset the gas disk's orbit in two ways. The magnetic fields could push the wind outward like a spring, or the force of the spinning disk could fling the wind away from the disk's center. Either disruption of the disk's orbit would cause some matter to spiral downward.

Miller's team appears to prefer the first scenario, says Roger Blandford of Stanford University. Blandford prefers the second, which he describes as an organized magnetic slingshot. "I suspect there are different answers in different types of systems," he says. "But on the face of it, this looks like a big advance observationally."

Though most astrophysicists expected magnetic fields to play a role in black holes, the finding is "very important evidence that what we thought before makes sense," says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "It's the first time there's clear evidence for wind coming off an accretion disk."

The finding could help astrophysicists understand the "complex give-and-take between galaxies and black holes," Miller says.