Awakening the Milky Way’s black hole
BY EMILY BALDWIN
ASTRONOMY NOW
Posted: April 16, 2008

Our galaxy’s central black hole let loose a powerful flare three centuries ago, say Japanese astronomers using NASA’s Chandra X-ray Observatory, Japan’s Suzaku and ASCA satellites and Europe’s XMM-Newton X-ray Observatory.

The quiescence of the Milky Way’s black hole, Sagittarius A*, located 26,000 light years away, has been a long-standing mystery, but the new studies suggest that it could have been a lot more active in the past. Sagittarius A* contains about four million times the mass of the Sun, yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies. "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."

This Chandra image shows our Galaxy's centre, with the location of the black hole, known as Sagittarius A*, or Sgr A*, labelled. Image: NASA/CXC/MIT/Frederick K. Baganoff et al.

The observations, collected between 1994 and 2005, reveal that clouds of gas near the central black hole brighten and fade quickly in X-ray light in response to to X-ray pulses emanating from regions just outside the black hole. These pulses take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, and once the pulses reach the cloud they react with the iron atoms, emitting X-rays, before returning to its normal brightness once the X-ray pulse has subsided. Usually the cloud responds to events that occurred 300 years earlier, but the Japanese astronomers were astounded to see that the brightness of a region just 10 light years across varied considerably in just five years, and attributed the observations to 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," 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."

This new study builds on previous research that pioneered the the light-echo technique. Last year, a team lead by Michael Muno, who is now based at the California Institute of Technology in Pasadena, used Chandra observations of X-ray light echoes to show that Sagittarius A* generated a powerful burst of X-rays about 50 years ago, about a dozen years before astronomers had satellites that could detect X-rays from outer space. "The outburst three centuries ago was 10 times brighter than the one we detected," says Muno.

Why Sagittarius A*’s activity varies so much still poses a big question to the astronomers working on this project, but there are several possibilities. According to Koyama, one possibility is that a supernova a few centuries ago 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.

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 the varying X-ray output from our galaxy's central black hole. Top left: ASCA (imaged 1994); top right: Chandra (2000); bottom left: XMM-Newton (2004); bottom right: Suzaku (2005). Images: JAXA, NASA/CXC, ESA