'Milky Way's dormant giant black hole lit up 300 years ago'

Washington, apr 16 (UNI) The black hole in the centre of Milky Way Galaxy let loose a powerful flare 300 years ago, astronomers have discovered.

The finding by a team of Japanese astronomers helps resolve a long-standing mystery: why is the Milky Way's black hole so dormant? ''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 realise that the black hole was far more active in the past. Perhaps it's just resting after a major outburst.'' The central black hole, known as Sagittarius A* contains about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies.

The observations, collected between 1994 and 2005, revealed that clouds of gas near the 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 10 years, we could trace back the black hole's activity 300 years ago,'' says 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 new study, which will appear in the Publications of the Astronomical Society of Japan, combines results from Japan's Suzaku and ASCA X-ray satellites, NASA's Chandra X-ray Observatory, and the European Space Agency's XMM-Newton X-ray Observatory.

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