Black Hole Grabs Planet-Sized Snack
This set of Chandra images shows evidence for a light echo generated by the Milky Way's supermassive black hole, a.k.a. Sagittarius A* (pronounced "A-star").
Credit: NASA/CXC/Caltech/M.Muno et al.

SEATTLE--An ancient X-ray outburst from the supermassive black hole at the center of our Milky Way galaxy caused surrounding gas clouds to glow brightly in a cosmic light show that is only now being detected.

The output likely involved the consumption of a snack equal in mass to the planet Mercury, researchers said here yesterday at the 209th meeting of the American Astronomical Society.

Called Sagittarius A*, the supermassive black hole at the Milky Way's core is located some 27,000 light-years away and has an estimated mass of about three million suns. It is surrounded by several massive iron-rich gas clouds that glow and emit their own X-rays when struck by photons or electrons [image].

No X-ray telescopes were in place when light from the black hole outburst first began to reach Earth about 60 years ago, but astronomers deduced the event based on "light echoes" from the clouds recently recorded by NASA's Chandra X-ray Observatory.

Echoes of a meal

Light echoes are reflections of light from a primary source--in this case a black hole outburst--bounced off another object such as dust or gas.

Observations by Chandra over several years reveal different areas of the gas clouds lighting up, as if a wave of energy were passing through them, illuminating different areas at a time.

Sagittarius A* is known to have produced smaller and more recent outbursts, but this latest one is estimated to have been about 1,000 times brighter and lasted about 1,000 times longer than the others. Evidence from other studies looking at light echoes originating outside the galactic plane suggests Sagittarius A* experiences an outburst every 100 years or so.

The researchers think the outburst was triggered by a small amount of matter falling into the black hole.

"It doesn't actually require a lot of mass falling into the black hole to produce this outburst," said Michael Muno of Caltech. "The mass would be equivalent to the planet Mercury."

As matter swirls around a black hole, it generates a large amount of friction, producing X-rays.

Nobody knows what the black hole actually devoured. Like other galactic black holes, Sagittarius A* is thought to be surrounded by a swirling disk of matter blown in by stellar winds. Muno speculates that every once in a while, something upsets the swirling disk, causing a hiccup in its motions that makes it dump its material onto the black hole, triggering an outburst.

"Another option is that there may be small clouds of gas and dust drifting around the galactic center and every once in awhile, one of those get close enough to the black hole," he said.

Strangely calm

For all its activity, however, Sagittarius A* is unusually faint for a galactic supermassive black hole. The answer may lie in its maturity. Our galaxy is several billion years old, and one idea is that most of the galaxy-building that can feed a black hole has already taken place.

A quasar, on the other hand, is typically a relatively young galactic setup involving a black hole that can outshine the combined starlight of its entire host galaxy.

The energy output of Sagittarius A* is nowhere close to this. Why our black hole is so dim is not entirely understood.

"This faintness implies that stars and gas rarely get close enough to the black hole to be in any danger," said study team member Frederick Baganoff of the Massachusetts Institute of Technology. "The huge appetite is there, but it's not being satisfied."