Black Hole Snacks
But finding it hasn't been easy. Light (by definition) can't escape a black hole, so the Milky Way's central monster has remained elusive.
Right: An artist's concept of matter swirling, like water down the drain of a bathtub, into a supermassive black hole. Courtesy of the Chandra X-ray Observatory's X-ray Astronomy Field Guide.
Undaunted, observers have been peering into the heart of the Milky Way for many years. The fast motions of stars and gas around the galactic center suggested that something very massive indeed was hidden there. But what? If it was a black hole, X-ray telescopes ought to detect a telltale glow from super-heated gas swirling into the hole -- what astronomers call an "accretion disk." Astronomers had spotted such X-ray emitting disks at the hearts of other galaxies, but not inside the Milky Way. It was a real puzzle.
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Now, they may have caught the black hole in the act of devouring something, too.
On October 26, 2000, Baganoff and colleagues were once again using Chandra to monitor the center of our galaxy when they recorded a powerful X-ray outburst. They were looking toward "Sagittarius A*," an intense radio source that astronomers believe is powered by the black hole. During a span of just a few minutes, X-ray emissions from Sagittarius A* became 45 times brighter than normal, before declining to pre-flare levels a few hours later.
"This is extremely exciting because it's the first time we have seen our own neighborhood supermassive black hole devour a chunk of material," said Baganoff. "It's as if the material there sent us a postcard just before it fell in."
The energy released in the flare corresponds to a sudden infall of material with about as much mass as a comet or an asteroid. The black hole literally gobbled something up! On the other hand, say scientists, the flare might have been caused by the reconnection of magnetic field lines near the black hole -- a process that also triggers solar flares on the Sun.
Above: This false-color image shows the central region of our Milky Way Galaxy as seen by Chandra. The bright, point-like source at the center of the image was produced by a huge X-ray flare that occurred in the vicinity of the supermassive black hole at the center of our galaxy. Credit: NASA/MIT/F. Baganoff et al. [more]
In either case, shock waves from the explosion would have accelerated electrons near the black hole to nearly the speed of light -- leading to the observed outburst of X-rays. A longer-term increase in radio emission was also recorded beginning around the time of the flare, indicating that, indeed, the production of high-energy electrons was increasing.
At the peak of the flare, the X-ray intensity dramatically dropped by a factor of five, then recovered, all within a 10-minute interval. Such fluctuations constrain the size of the emitting region to be no larger than about 20 times the size of the event horizon -- the one-way membrane around a black hole predicted by Einstein's theory of relativity.
Below: An artist's illustration of the orbiting Chandra X-Ray Observatory. Credit: TRW.
"This signal comes from closer to the event horizon of our galaxy's supermassive black hole than any that we have ever received before," agreed Baganoff. Indeed, X-ray observations of Sagittarius A* seem to offer a unique way to probe activity very close to such black holes where space and time itself are twisted.
It's a prospect that thrills astronomers who have been waiting a long time for such data. Consider that the first discussions of black holes -- by the French physicist Pierre Laplace and the English philosopher John Michell -- date back to the 18th century. But until recently we couldn't even find the biggest one in our own galaxy! Thanks to Chandra's sensitive X-ray cameras, however, black holes are finally revealing their secrets, a well-earned reward for patient astronomers.
Editor's Note: This week X-ray astronomers are celebrating Chandra's 2nd successful year of discoveries as well as the good news from NASA headquarters that the Observatory's mission has been doubled from 5 to 10 years. For more information visit http://chandra.nasa.gov or http://chandra.harvard.edu.
Web Links
X-Rays - Another Form of Light -- the basics of X-rays from Harvard's Chandra X-ray Observatory home page
Black Holes and Accretion Disks -- A tutorial from the Harvard CXO Field Guide to X-ray Astronomy
Chandra X-ray Observatory Center -- Gateway to the Universe of X-ray astronomy, for journalists, students and scientists.
NASA's Chandra X-ray Observatory marks two years of important discoveries -- A Marshall Space Flight Center press release.
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