Washington, April 16 : A team of Japanese astronomers has discovered that our galaxy's central black hole, which is in a state of slumber currently, had let loose a powerful flare three centuries ago.
Discovered by the team of astronomers using NASA, Japanese, and European X-ray satellites, the black hole, known as Sagittarius A, is humongous, containing about 4 million times the mass of our Sun.
Despite this fact, the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies.
According to team leader Tatsuya Inui of Kyoto University in Japan, "We have wondered why the Milky Way's black hole appears to be a slumbering giant."
"But now, we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst," he added.
The team's new study 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.
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.
Amazingly, a region in Sagittarius B2 only 10 light-years across varied considerably in brightness in just 5 years. These brightenings are known as light echoes.
By resolving the X-ray spectral line from iron, Suzaku's observations were crucial for eliminating the possibility that subatomic particles caused the light echoes.
"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," he added.