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Black hole had 18th century feeding frenzy

About 300 years ago, the black hole lurking in the heart of the Milky Way woke from hibernation and entered a feeding frenzy, triggering a cascade of x-rays that reverberated off nearby clouds, researchers say.

The energy released by the event was so intense that echoes remain etched in an interstellar cloud today.

The findings, reported by Japanese astronomers, may explain why our galaxy's black hole, known as Sagittarius A*, is so laid-back compared to similarly sized monsters in other galaxies.

"Perhaps it's just resting after a major outburst," says Kyoto University's Tatsuya Inui, lead author of a paper in the 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.

The key to unravelling 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 US, 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," says co-investigator Professor Katsuji Koyama.

"The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare."

More recent outburst

Last year, a team of scientists headed by Dr Michael Muno at the California Institute of Technology discovered a fainter but more recent outburst from Sagittarius A*, which is between three and four million times as massive as our sun.

It too was discovered by studying x-rays reflected off a gas cloud. During that outburst, a mass the size of the planet Mercury was devoured.

"It has been 50 years or so since the black hole had its last decent meal," Muno says.

If x-ray sensors had been around then, the area around the black hole would have been 100,000 times brighter than it is today.

Feeding habits

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 towards the black hole.