By Ron Cowen
A little more than 26,000 years ago, a sleeping giant at the center of our galaxy suddenly awoke, spewed several pulses of X-rays and went back into hibernation.
That’s the conclusion of a long-term X-ray study of large gas clouds near the Milky Way’s central, supermassive black hole — a 4-million-solar-mass monster dubbed Sagittarius A*. Data collected from four spacecraft between 1994 and 2005 show that X-ray pulses, apparently shot into space by material just before it spiraled into the central black hole, caused the clouds to rapidly fluoresce and then fade. The intensity of the brightening suggests that the black hole, which now appears unusually quiescent, was once much more active. Material caught in its grip glowed a million times more brightly at X-ray wavelengths and may have been one of the brightest X-ray sources in the sky.
Because the clouds lie 300 light-years from the black hole, their fluorescence reveals the activity of the supermassive black hole 300 years earlier in time, notes Katsuji Koyama of Kyoto University in Japan.
Koyama and his Kyoto colleagues report the findings in an upcoming Publications of the Astronomical Society of Japan.
Spacecraft including NASA’s Chandra X-ray Observatory had previously found similar hints that our galaxy’s central black hole was once more active. But it took Japan’s Suzaku X-ray observatory to prove that the clouds near the black hole had temporarily brightened due to fluorescence, a process in which atoms — iron atoms in this case — generate X-rays almost immediately in response to an incoming X-ray pulse. The quick response proves that the brightening of the clouds traces the most recent activity of the supermassive black hole, rather than activity that may have occurred up to thousands of years earlier.
Astronomers see the region around the black hole as it appeared 26,000 years ago, since the galaxy’s center lies 26,000 light-years from Earth.
It’s still a mystery why the Milky Way’s central black hole is one of the most quiescent known and why the giant suddenly — but fleetingly — increased its X-ray output in the recent past. One possibility is that it went on a feeding frenzy, gorging on a temporary surplus of gas. This increase in food supply might have been created when a nearby supernova explosion hurled gas toward the black hole, suggests X-ray astronomer Richard Mushotzky of NASA’s Goddard Space Flight Center in Greenbelt, Md. He notes that as astronomers attempt to examine X-ray fluorescence from clouds that lie even farther from the supermassive black hole, they may be able to probe the activity of the giant even farther back in time.
