The black hole of the Milky Way

Japanese astronomers has discovered that the black hole in the center of our galaxy unleashed a powerful blast for three centuries, thanks to observations with the ESA observatory XMM-Newton and other X-ray satellites and NASA Japanese.  This discovery helps solve a longstanding mystery: why the black hole of the Milky Way is so inactive.  This black hole, known as Sagittarius A-star (A *), is truly massive, contains about four million times the mass of our Sun, the energy radiated from its surroundings is billions of times weaker than the emitted from central black holes in other galaxies. 

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

The observations, made ​​between 1994 and 2005, revealed that clouds of gas near the central black hole brightened and faded quickly in X-Ray in response to X-ray pulses emanating from just outside the black hole.  When gas spirals into the black hole is heated to millions of degrees and emits X-rays  As more matter piles up near the black hole, the X-ray emission becomes larger.

These X-ray pulses need about 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 could have been seen for 300 years from Earth .

When X-rays reach the cloud collide with iron atoms, kicking out electrons near the atomic nucleus.  When electrons from farther out fill in these gaps, the iron atoms emit X-rays  Once the X-ray pulse has passed through the cloud fades to its normal brightness.

Surprisingly, a region in Sagittarius B2 only 10 light-years across, varied considerably in brightness in just five years.  These brightenings are known as "light echoes".  By analyzing the spectral line from iron X-ray observations from Suzaku for eliminating the possibility that these echoes were produced by subatomic particles.

"By observing how this cloud lit up and faded over 10 years we have been able to reconstruct the history of black hole's activity 300 years ago," said Katsuji Koyama, a team member at the University of Kyoto.  "The black hole was a million times brighter three centuries ago.  It must have been incredibly powerful. "

This new study builds on research by several groups who pioneered the technique of "light echo".  Last year a team led by Michael Muno, who now works at the California Institute of Technology (USA), used the observations of light echoes obtained by Chandra X-ray to show that Sagittarius A * generated a powerful X-ray emission For five decades, about a dozen years before astronomers they had satellites that could detect X-rays from outer space.  "The explosion produced three centuries years was 10 times brighter than the emission that we observe," says Muno.

The center of the galaxy is about 26 000 light-years from Earth, meaning we see events as they occurred 26,000 years ago.  Astronomers do not yet have a detailed explanation of why Sagittarius A * varies so much activity level.  One possibility, says Koyama, is that a supernova a few centuries ago expel a cloud of gas that fell into the black hole, this was a huge supply of matter black hole awoke from its slumber and produced the potent issue.

The new study, appearing in the Publications of the Astronomical Society of Japan, combines results obtained by X-ray satellites Japanese Suzaku and ASCA , the X-ray Observatory and NASA's Chandra X-ray observatory of ESA XMM -Newton .  The article that presents the results entitled "Time Variability of the Neutral Iron Lines from the Sgr B2 Region and Its Implication of a Past Outburst of Sgr A *", by T.  Inui, K.  Koyama, H.  And T. Matsumoto  Tsuru.

Launched in 2005, Suzaku is the fifth Japanese satellite in a series of observatories dedicated to the study of cosmic X-ray sources and is managed by the Japan Aerospace Exploration Agency (JAXA).  This mission is a collaboration between Japanese universities and institutions and NASA Goddard .

XMM-Newton, the X-ray orbiting observatory of ESA, is the largest scientific satellite ever built in Europe.  The mirrors of the telescope are the most sensitive ever developed, and its sophisticated detectors can see much more than any previous X-ray satellite.