Giant black holes are always hungry, but they're also patient. Thousands of years can pass between a black hole's major meals: stars that get torn to shreds when they venture too close. Astronomers now think they've spotted one of these gas-gulping frenzies in the act. The discovery may explain how black holes at the heart of galaxies--including the one inside the Milky Way--keep growing as they age.
The cores of nearly all galaxies harbor supermassive black holes. These objects contain millions or even billions of times as much mass as our sun. Their powerful gravitational fields force nearby stars to orbit rapidly, although most stars avoid plunging into the holes. Every so often, an encounter between two stars sends one of them diving closer. At that range the black hole's intense gravity ought to rend the star into a stream of gas.
Now, astronomers have their strongest evidence of such an event. In 1992, a remote galaxy called RX J1242-11 erupted in a flare of x-rays as energetic as a supernova explosion. By March 2001, the x-rays had faded by a factor of 200 but were still detectable. NASA's Chandra X-ray Observatory pinpointed the flare's origin at the galaxy's core, and the European Space Agency's XMM-Newton satellite spotted powerful x-rays covering a wide range of wavelengths. That pattern closely matches the expected signature of gas whirling in a blazing hot disk just before disappearing into a black hole, says astronomer Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, leader of the international team.
The team estimates that just a few percent of the star's gas--still a hefty ration--vanished into the hole. The doomed star's momentum flung the rest of the gas back into the galaxy, the researchers believe. They reported their discovery on 18 February at a NASA news briefing in Washington, D.C.
"This is fantastic stuff," gushes astronomer Alex Filippenko of the University of California, Berkeley. Using the team's methods, he says, astronomers can now check how frequently such tidal disruptions occur elsewhere. Supermassive black holes in quiet galaxies might wait between 10,000 years and 1 million years for each stellar meal, Filippenko notes. Even so, he suspects the rare events are the main way many black holes put on weight, because mature galaxies like the Milky Way have little extra gas at their cores.
Related sites
Animations
and other illustrations of black hole and star
X-ray
research on the Milky Way's supermassive black hole
Chandra X-ray
Observatory
XMM-Newton Observatory
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