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The Milky Way’s Slumbering Giant Awakes
Xray image of Sagittarius A black hole This Chandra image shows our Galaxy’s center. The location of the black hole, known as Sagittarius A*, or Sgr A* for short, is arrowed. Credit: NASA/CXC/MIT/Frederick K. Baganoff et al.
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At the center of our Milky Way Galaxy lies a monster: a giant black hole that contains about 4 million times more material than our sun. Yet it’s a sleeping giant. Compared to the giant black holes in the centers of other galaxies, our black hole is strangely quiet.

But a team of Japanese astronomers may have helped solve the mystery. By using four satellites that catch X-rays from outer space, they have found evidence that our black hole wasn’t so quiet 300 years ago. They have shown that the black hole let loose a powerful outburst of X-ray light.

"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."

The black hole itself is known as Sagittarius A* (pronounced "A-star"), for its location in the constellation Sagittarius. Normally, the black hole is quiet, producing billions of times less energy than giant black holes in other galaxies. But according to Inui and his colleagues, the black hole must have produced an incredible burst of X-ray light three centuries ago. They made this discovery by noticing a strange effect known as "light echoes."

Light echoes are similar to the sound echoes we hear when sound waves reverberate in a room or valley. In the case of light echoes, the X-rays produced by the giant outburst have been racing outward across trillions of miles of space at the speed of light. Three hundred years later, they have traveled far enough that they reach a giant gas cloud known as Sagittarius B2. Once they penetrate this cloud, they heat up the gas, and cause it to glow brightly in X-rays. But once the X-rays pass through the cloud, it cools down, and its brightness fades back to normal.

Four Xray images of Sagittarius B2 Four X-ray satellites imaged a small region in the gas cloud Sagittarius B2, and saw pockets brighten and fade over the course of nearly 12 years. These light echoes are caused by varying X-ray output from our galaxy’s central black hole. Credits: ASCA and Suzaku: JAXA; Chandra: NASA/CXC; XMM-Newton: ESA.
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So Sagittarius B2 acts like a giant mirror. The light echoes inside the cloud give astronomers a record of the black hole’s energy output 300 years earlier.

By using 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, Inui’s team could observe the behavior of the cloud between 1994 and 2005.

"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."

It takes light from the Milky Way Galaxy’s center about 26,000 years to reach Earth, so when astronomers observe the black hole and the gas cloud, they are actually seeing events that took place 26,000 years ago. At that time, Earth was still plunged in the last ice age, and humans were living in caves.

Astronomers don’t know why Sagittarius A* produced such a powerful flare three centuries ago. One possibility, says Koyama, is that a giant star exploded. The blast wave from the explosion plowed up gas and swept it into the black hole, leading to a temporary feeding frenzy that awoke the black hole from its slumber and produced the giant flare.

Launched in 2005, Suzaku is the fifth in a series of Japanese satellites devoted to studying celestial X-ray sources and is managed by the Japan Aerospace Exploration Agency. This mission is a collaborative effort between Japanese universities and institutions and NASA Goddard.
Robert Naeye
Goddard Space Flight Center