Magnetism, Not Just Gravity, Makes Black Holes Suck, Study Says

Sean Markey
for National Geographic News
June 21, 2006
Black holes have a reputation for being incredibly dense, sucking in all the light and matter around them thanks to their strong gravitational fields.

But new research suggests it's actually magnetic wind, not just gravity, that makes these massive collapsed stars so impossible to resist.

According to a study appearing in tomorrow's issue of the journal Nature, magnetic fields help matter flow into black holes.

Beyond a certain distance, "matter can orbit black holes pretty much as it would a normal star or in the way things orbit the Earth," said lead author Jon M. Miller of the University of Michigan in Ann Arbor.

"Things already in a stable orbit tend to stay in a stable orbit. We call that angular momentum."

But if a black hole has a neighboring star in close orbit, gas from that star is sucked in. The displaced matter forms a thin bright disk that spirals around the black hole and slowly falls into it.

Some force other than gravity must therefore be exerting itself within the halo of stellar gas—and Miller's team thinks the culprit is magnetic friction.

Mighty Wind

To date, astronomers have identified 20 black holes in the Milky Way and suspect there may be millions more in our galactic neighborhood.

Scientists know a good deal about black holes, including their instrumental role in galaxy formation (related news: "Supermassive Black Hole at Center of Milky Way, Study Hints").

But how the objects seem to defy the law of angular momentum has been a long-standing mystery.

Using NASA's orbiting Chandra X-ray Observatory (gallery: x-ray images from Chandra), Miller's team focused their attention on GRO J1655-40.

This poetic name belongs to a nearby star and small black hole, or binary system, locked in mutual orbit in our galaxy.

For some time now, GRO J1655-40's black hole, which has seven times the mass of our sun, has been sucking superhot gas from the atmosphere of its companion star.

Via the Chandra satellite, Miller and his colleagues observed a wind of electrically charged particles emanating from GRO J1655-40.

The team believes the wind is produced by the same magnetic field that funnels matter into the black hole.

Other possible causes, such as heat or radiation pressure, are too weak to produce such a mighty wind, the team's data suggest.

The magnetic field itself would explain why the orbit of the disk around GRO J1655-40's black hole decays.

"Magnetic fields are going to cause friction in that gas, and that will [overcome angular momentum and] cause the gas to slowly spiral inward to the black hole," Miller said.

X-Ray Vision

The researchers say such friction also explains another phenomenon: why black holes actively absorbing stellar gas can be the brightest x-ray objects in the universe.

Friction causes the disks of stellar gas surrounding black holes to superheat and glow, Miller says.

"It [can be] so bright in x-rays that its output begins to rival the summed x-ray emission of the entire rest of the Milky Way," he said.

Daniel Proga, an assistant professor of physics at the University of Nevada, Las Vegas, says the magnetic-wind argument relies mostly on indirect evidence, and not every scientist will find it convincing.

"But this is the best that can be done at the moment," Proga wrote in an accompanying Nature news article.

"Many astronomers will be motivated by such work to develop and explore this promising model," he continued.

Study co-author Miller says the fine resolution of NASA's Chandra X-ray Observatory, launched in 1999, enabled his team to collect the data for their revolutionary theory.

"Without the instrumental advances that have been realized on Chandra, work of this kind would really be impossible," he said.

The astrophysicist says a planned NASA x-ray satellite, called Constellation X, will enable similar scientific discoveries in the future.

"It will allow us to do a lot of this really very detailed work [on] far more distant black holes."

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