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Ancient outburst of galaxy’s black hole observed

Astronomers have recently spotted echoes of a giant flash from 300 years ago in a large gas cloud called Sagittarius B2.
Image: Sagittarius A*
NASA/CXC/MIT/F.K. Baganoff et al
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There is a slumbering matter-munching monster in the middle of our galaxy, and every once in a while, this black hole flares up and releases plumes of X-rays.

Matter is constantly falling into the Milky Way's central supermassive black hole, but sometimes enough builds up and gets hot enough to release a big flash. For instance, if humans had an X-ray observatory 300 years ago, we would have seen a giant flash caused by a clump of gas heating up as it fell toward the black hole.

Although we missed this mighty burst, astronomers have recently spotted echoes of it in a large gas cloud called Sagittarius B2. The X-rays took 300 years to travel from the central black hole to the cloud, and when they arrived, they collided with iron atoms, causing them to emit X-rays.

A team of Japanese astronomers observed the cloud in 1994 with the ASCA X-ray satellite and then again in 2000 with NASA's Chandra X-ray Observatory, in 2004 with the European Space Agency's XMM-Newton X-ray Observatory, and in 2005 with Japan's Suzaku X-ray satellite.

Today they announced the results of their investigation.

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"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 team member Katsuji Koyama of Kyoto University. "The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare."

The discovery of this flare may help explain why our galaxy's central black hole, called Sagittarius A* (pronounced "A-star"), seems to be less active than those in other galaxies.

"We have wondered why the Milky Way's black hole appears to be a slumbering giant," says team leader Tatsuya Inui of Kyoto University in Japan. "But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst."

Since the center of our galaxy is 26,000 light-years from Earth, both the X-ray flash and the echoes we're seeing now in Sagittarius B2 actually occurred a long time ago, roughly 26,300 years back.

The astronomers will detail their findings in an upcoming issue of the journal Publications of the Astronomical Society of Japan.