Illustration courtesy M. Weiss, CXC/NASA
An x-ray view of Sagittarius A*. Image courtesy F. Baganoff et al, MIT/CXC/NASA.
Published February 9, 2012
Dubbed Sagittarius A* (pronounced "A star"), the monster black hole lies 26,000 light-years away in the galactic center. The black hole is surrounded by an accretion disk—a swirling ring of superheated gases—which spews radiation as matter is consumed.
But astronomers have also seen bright x-ray flares around the black hole since the start of NASA's Chandra X-ray Observatory mission in 1999. In addition, the flares have been seen in infrared wavelengths by the European Southern Observatory's Very Large Telescope in Chile.
Occurring almost daily, the flares can last for up to an hour and can be up to a hundred times as bright as the black hole's typical output.
Now, a team of astronomers led by Kastytis Zubovas of the University of Leicester in the U.K. thinks the flares can be attributed to the death throes of asteroids that are being drawn into the black hole's accretion disk.
According to computer models based on Chandra data, there may be a cloud containing a few trillion asteroids orbiting Sagittarius A* at least a hundred million miles (160 million kilometers) from the edge of the accretion disk.
As the asteroids' orbits get perturbed by the black hole's gravity, the objects fall inward and get ripped apart by tidal forces. The rocky remains then encounter hot gases within the accretion disk and vaporize—much in the same way that a meteor disintegrates in Earth's atmosphere.
When this occurs, Earthly observatories see flares of brilliant radiation.
Black Hole "Keeps on Bubbling Away"
It's possible that the rocky buffet is made up of asteroids stolen from nearby stars, according to Peter Edmonds, an astronomer at the Chandra X-ray Center in Cambridge, Massachusetts, who was not involved in the new study.
"The asteroids could have been pulled off solar systems and then pulled into a cloud surrounding the black hole," Edmonds told National Geographic News.
The research team estimates that only asteroids at least 6 miles (10 kilometers) wide create flares bright enough to be seen from Earth.
But with trillions of such space snacks in the cloud, it's assumed that smaller asteroids are also being gobbled up on a regular basis, resulting in dimmer, unseen flares.
And this supermassive black hole buffet is likely nothing new: "It's logical to assume that these flares have been going on for a long time," Edmonds said.
This assumption is backed up by the presence of "light echoes" in the vicinity of the black hole. (Related: "'Light Echo' Helps Solve Supernova Mystery.")
Past flares—especially brighter ones likely caused by the black hole gobbling up larger objects such as planets—can create emissions that resonate off localized clouds of gas and dust, hinting at the flares' original radiance for hundreds of years.
And even now Sagittarius A* is showing no signs of curbing its appetite, Edmonds added.
"It just keeps flaring ... it just keeps on bubbling away."
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