Astrophysicists have caught our galaxy belching after a small meal, all but clinching the case for a black hole at the center of the Milky Way. The burp--really a flare of x-rays from the suspected hole--came last year and is described in a report in the 6 September issue of Nature. Future flares will reveal much about how the black hole feeds, experts say.
The existence of the Milky Way's black hole, despite its estimated mass of 2.6 million suns, is difficult to pin down. Because our black hole is calm, astronomers relied on indirect evidence, like the rapidly whirling orbits of nearby stars (Science, 7 January 2000, p. 65). Then in 1999, astrophysicists detected a steady buzz of x-rays flowing from an object called Sagittarius A*, a radio beacon at the galaxy's core--additional evidence for a black hole.
Now, a burst of energy strengthens the idea. The Chandra X-ray Observatory peered at Sagittarius A* for 10 hours last October and caught a 2-hour spike of unusually bright x-rays. The flare was erratic, which means that the x-rays must come from a region no wider than the distance between Earth and the sun, says astrophysicist Frederick Baganoff of the Massachusetts Institute of Technology in Cambridge. "That's exciting," says Baganoff, "because it constrains the emission to a zone about 20 times the size of the black hole's event horizon," the boundary beyond which light cannot escape.
A comet-sized clump of hot gas falling into the black hole might have spewed the x-rays, Baganoff says. It also could have come from tangled magnetic fields around the hole. Chandra will stare at Sagittarius A* for a full week next April to try to see several more flares, Baganoff says. Simultaneous worldwide observations with radio telescopes should help theorists determine just how the black hole releases energy.
Observers say this x-ray flare makes the black hole virtually a certainty. Chandra's tight size constraint rules out other explanations, such as a nest of compact dead stars, says astrophysicist Fulvio Melia of the University of Arizona, Tucson. "The only thing missing now is to actually take a photograph of this dark pit, the shadow produced by the black hole," Melia says. That, he says, should happen by 2010 as researchers develop a worldwide network of sensitive radio telescopes in the millimeter band.
Related sites
Chandra X-ray Observatory
Center
Background
about earlier discovery of x-rays from galaxy's black
hole
Technical report
on previous Chandra observations of Sagittarius A*
NASA
article on x-ray flare
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