New York: Astronomers in Japan claim to have discovered that the Milky Way's giant black hole awoke from slumber three centuries ago.

Using American, Japanese and European satellites, they have solved the long-standing mystery why is our galaxy's black hole, Sagittarius A, containing four million times the mass of our Sun, is so quiescent, the NASA said.

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

The observations, collected between 1994 and 2005, revealed that clouds of gas near the central black hole brightened and faded quickly in X-ray light as they responded to X-ray pulses emanating from just outside the black hole.

When gas spirals inward toward the black hole, it heats up to millions of degrees and emits X-rays. As more and more matter piles up near the black hole, the greater the X-ray output. These X-ray pulses take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, so the cloud responds to events that occurred 300 years earlier.

When the X-rays reach the cloud, they collide with iron atoms, kicking out electrons that are close to the atomic nucleus. When electrons from farther out fill in these gaps, the iron atoms emit X-rays. But after the X-ray pulse passes through, the cloud fades to its normal brightness.

"By observing how this cloud lit up and faded over ten years, we could trace back the black hole's activity 300 years ago. The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare," team member Katsuji Koyama said.

Source : PTI