The Milky Way's Monster Black Hole Awakes! Fired Out a Massive Flare 300 Years Ago

Astronomers have long wondered why it was that the super-massive black hole in the center of our galaxy was relatively quiet. Known as Sagittarius A*, a massive hole, containing about 4 million times the mass of our Sun. Yet, despite its size, the energy radiated from its surroundings is billions of times weaker than black holes at the core of other galaxies.



This new X-ray image of the center of the Milky Way above is dominated by Sagittarius A. The black hole and several massive young stars in the chaotic region are creating the surrounding haze of superheated gas that shows up in 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." Tatsuya Inui is part of a team that used 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, to determine the history of our 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.

It turns out that, approximately 300 years ago, Sagittarius A* let loose, expelling a massive energy flare. Data taken from 1994 to 2005 revealed that clouds of gas near the central black hole, known as Sagittarius B2, brightened and faded quickly in X-ray light. The X-rays were emanating from just outside the black hole, created by the buildup of matter piling up outside the black hole, which subsequently heats up and expels X-rays. These pulses of X-ray take 300 years to traverse the distance between Sagittarius A* and Sagittarius B2, so that when we witness something happening in the cloud, it is responding to something that happened 300 years ago.

Amazingly for us, in a rare occurrence of perfect cosmic timing, a region in Sagittarius B2, only 10 light-years across varied dramatically in 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."

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.