WASHINGTON — A hyperactive black hole called Old Faithful that is gobbling up a star and belching huge geysers of gas is giving astronomers their first insight into one of the major mysteries of the heavens.
Researchers have long wondered how black holes, those cosmic vacuum cleaners that suck up everything in their vicinity, could spit out enormous globs of matter and propel them to speeds up to 90% of the speed of light. Now they believe they have clear signals that the hole lassos nearby matter into a flat disk, then periodically "flips" the disk into space.
Sitting in the constellation Aquila, the relatively small hole erupts with fountains of electrically charged particles, Old Faithful-style, every 30 minutes. Each flare ejects 100 trillion tons of gas at nearly light speed.
Seconds before the flare, the X-ray-emitting disk of gas that normally surrounds the black hole completely disappears. The hole "grabs the inner disk and throws it out at high speed," Caltech astronomer Stephen Eikenberry reported Wednesday at the American Astronomical Society meeting here. "After decades of looking, we've finally seen" the mechanism that produces the high-speed jets.
Black holes are extreme puckers in the fabric of space-time caused by runaway gravity. Until only recently thought too bizarre to exist, they are now accepted as a diverse family of objects, ranging from the size of subatomic particles to millions of times the mass of our sun.
With only 10 to 30 times the mass of the sun, the little hole in Aquila nevertheless easily stole astronomers' attention from a far more massive and sedate black hole at the center of the Milky Way, also discussed at the meeting.
The rapid motions of stars whipping around the center of the galaxy have long suggested that a massive black hole might be shrouded in the thick gas and dust there. But scientists couldn't be certain. New measurements zooming in on the center, some 86,000 light-years away, reveal that 2.6 million times the mass of the sun is concentrated into a relatively tiny area no more then five light-days across.
"This object has to be a massive black hole," concluded one of the investigators, Andreas Eckart of the Max Planck Institute in Germany.
Since black holes trap light, as well as everything else, they can't be seen directly. However, matter falling into the hole spirals around like water going down the drain, and emits high energy radiation in the process.
The massive hole in the center of our galaxy--if that's what it is--only dribbles small amounts of energy, although researchers don't know why. Perhaps fierce stellar winds created by newborn stars blow away any matter that could fall into the hole, or perhaps the matter just silently slips in.
"It's an open question," said Mark Reid of the Harvard-Smithsonian Center for Astrophysics, another of the researchers. "There's a lot we don't know about black holes and radiation."
Even more puzzling than the lethargy of this hole are the wild "mood swings" of the smaller hole in Aquila, some 40,000 light-years away. As the all-consuming gravity of the hole rips off the outer layers of its unseen companion, it creates a cascade of matter spiraling toward the center, according to MIT astronomer Ronald Remillard. Astronomers knew that this disc of spiraling matter periodically empties and then refills, he explained. But while the amount of matter lost would rival Mt. Everest, astronomers couldn't tell "whether it went into the black hole or got ejected," he said.
Independently, other astronomers using different methods had seen jets of matter squirted out of black holes. Now, for the first time, they see that the emptying of the disk and ejection of matter are connected.
"The disk disappears," said Eikenberry. "A few seconds later, there's an infrared flare."
Old Faithful does not always live up to its name, however. In fact, it seemed to be in hibernation before 1992, when astronomers first noticed its strange behavior. At other times, it flickers or flares brightly.
"There are many moods to this source," said Remillard. "Before 1992, it was invisible. Now it's the wildest thing in the sky."
When it's in the right mood, however, it sticks to a fairly periodic schedule of flares every 20 to 30 minutes.
Astronomers do not understand what makes the black hole erupt on such a regular schedule, then lapse into a pattern of fits and starts. Gravity, which dominates all large-scale events in the universe, ultimately fuels the behavior of black holes. But gravity rules ocean waves as well, and they are far from consistent.
The motions of the Earth, moon, sun and atmosphere all play roles in molding their behavior, said Jean Swank of NASA's Goddard Space Flight Center in Maryland. In the same way, the black hole and the companion star pull on each other and affect each other in unpredictable ways. "There's a complex relationship between the star and the black hole," Remillard said.
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A Celestrial Geyser
A relatively small black hole in the constellation Aquilla erupts with fountains of electrically charged particles, Old Faithful-style, every 30 minutes, scientists said Wednesday. Seconds before erupting, the black hole grabs the X-ray-emitting disk that normally surrounds it, and then throws it out at high speed.
(1) The black hole pulls hot gas from the surface of an nearby companion star.
(2) The hot gas builds into an "accretion disk" around the black hole.
(3) At half-hour intervals, the accumulated matter is hurled outward in opposite directions from the black hole in jets moving at nearly the speed of light.
\o7 Source: NASA\f7