April 17, 2008 -- About 300 years ago, the black hole lurking in the heart of the Milky Way galaxy woke from hibernation and entered a feeding frenzy, triggering a cascade of X-rays that reverberated off nearby clouds. The energy released by the event was so intense, echoes remain etched in an interstellar cloud today.
The findings, reported by a team of Japanese astronomers, may explain why our galaxy's black hole, known as Sagittarius A-star, is so laid-back, compared to similarly sized monsters in other galaxies.
"Perhaps it's just resting after a major outburst," said Kyoto University's Tatsuya Inui, lead author of a paper in an upcoming issue of Publications of the Astronomical Society of Japan.
The scientists combined data from a trio of probes scouting the cosmos for X-rays. Data taken between 1994 and 2005 revealed a quick-lived but intense flash of X-rays from a large cloud known as Sagittarius B2, which lies 300 light-years from the Milky Way’s black hole.
Using a technique pioneered only a year earlier, the researchers showed the echoing light waves emanated from the black hole, rather than being triggered by subatomic particles, for example.
The key to unraveling the X-ray source was the detection of iron molecules in the spectral data, which shows the chemistry of areas the light had passed through.
Scientists believe the black hole's frenzy spewed out X-rays that blasted electrons off iron molecules in the cloud. The shift of electrons to and fro triggered sensors in X-ray detectors on U.S., European and Japanese satellites.
"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole's activity 300 years ago," said co-investigator Katsuji Koyama, also of Kyoto University. "The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare."
Last year, a team of scientists headed by Michael Muno at the California Institute of Technology discovered a fainter but more recent outburst from Satittarius A-star, the Milky Way's black hole, which is between three and four million times as massive as our sun.
During that outburst, which, like the new finding, was discovered by studying X-rays reflected off a gas cloud, a mass the size of the planet Mercury was devoured.
If X-ray sensors had been around 50 years ago, the area around the black hole would have been 100,000 times brighter than it is today.
"It has been 50 years or so since the black hole had its last decent meal," Muro said.
Details of the black hole's feeding habits are unclear. One theory is that the black hole pulls in matter from the winds of nearby young stars. The burst 300 years ago could have been caused by a supernova explosion that blasted material toward the black hole.
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