Black Holes Rely on Magnetism, Researchers Say

Black holes have a voracious appetite for matter, which often swirls around them in a wide disk. But gravity alone is not enough to draw that matter into the maw. Scientists say they may have solved a longstanding mystery about how that material is dragged from its orbit around the black hole. The answer involves magnetism, rather than gravity.

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Here's another story from the pages of the journal Nature and it's about something much more powerful than even a strong earthquake. Astronomers say that they may have solved a long standing mystery about how black holes managed to eat the matter that surrounds them. They have a huge appetite, but a black hole's amazing gravitational tug is not enough to slurp up matter. New observations suggest that black holes rely on magnetism to devour the gas clouds that form around them.

NPR's Richard Harris has the story.

RICHARD HARRIS reporting:

Black holes have such a strong gravitational tug not even light can escape. So, it's not obvious they'd need magnetism or any other force to feed themselves. John Miller at the University of Michigan says, think of it this way - the Earth is attracted to the sun by gravity, but we orbit the sun indefinitely. We never get drawn any closer.

Dr. JOHN MILLER (University of Michigan): If our sun suddenly turned into a black hole, the Earth would orbit it as though nothing would happen, apart from the fact that we'd have no nice yellow sun in the sky.

HARRIS: Gravity is simply not enough to tear something out of orbit. So, astronomers have wondered why the discs of material that form around black holes ultimately get gobbled up. To solve this mystery, Miller and his colleagues studied a nearby black hole. It's about as massive as our own sun, but with another star orbiting extremely close by.

Dr. MILLER: The two bodies, a normal star and the black hole, continue to orbit each other. And every so often, the black hole manages to draw off enough of its companion's atmosphere that the gas slowly flows onto the black hole, and as it does this, it heats up and becomes very bright.

HARRIS: This light is largely x-rays. The astronomers studied them using NASA's Chandra X-ray Observatory, which is orbiting the Earth. This particular black hole flared up for the first time in seven years. And when it did so, Chandra got a close look at the nature of the gases swirling all around. The x-rays told Miller and his colleagues how the gas was behaving. And from that they conclude that it's being influenced by magnetic forces.

Dr. MILLER: What magnetic forces internal to this orbiting gas do is provide some friction.

HARRIS: And just as the friction of bicycle brakes slows the wheels, the friction caused by the magnetic fields slows material in the disc. Once the material slows down, it can and it does fall into the black hole.

Dr. MILLER: And since this is a frictional force, it generates vast quantities of heat in the disc, causing the disc to glow brightly and that's why we see these huge flare ups in stellar mass black holes.

HARRIS: You can think of that flare up as the burp as the black hole swallows.

Dr. MITCH BEGELMAN (University of Colorado): What surprised me was the low speed of the gas.

HARRIS: Mitch Begelman at the University of Colorado has spent a lot of time trying to understand why black holes are such picky eaters on the one hand and so dazzling bright on the other when they do manage to gulp down matter. He says, the new measurements suggest the magnetic fields are fairly dainty in the way they toss some gases away as they consume others.

Dr. BEGELMAN: It's as if instead of being flung out into space quite violently, it's kind of riding a slow elevator out into space. That's what I found so surprising.

HARRIS: He expects the finding will apply to some of the most important objects in the universe, super massive black holes that live in the hearts of galaxies and influence the nature of those galaxies to a surprising extent.

Dr. BEGELMAN: Black holes really are in some sense an architect of galaxies. And the magnetic fields are the tools by which the architects can their building.

HARRIS: Eventually, John Miller hopes the next generation x-ray observatory will be able to look directly at those much more distant and much more powerful black holes.

Richard Harris, NPR News.

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