The giant black hole at the centre of the Milky Way may be vaporizing and devouring asteroids, which could explain the frequent flares observed, according to astronomers using data from NASA's Chandra X-ray Observatory. The team publish their results in the journal Monthly Notices of the Royal Astronomical Society.

An artist's impression of an asteroid approaching the black hole at the centre of the Milky Way. Credit: NASA / CXC / M. WeissFor several years Chandra has detected X-ray flares about once a day from the supermassive black hole known as Sagittarius A*, or "Sgr A*" for short. The flares last a few hours with brightness ranging from a few times to nearly one hundred times that of the black hole's regular output. The flares also have been seen in infrared data from ESO's Very Large Telescope in Chile.

"People have had doubts about whether asteroids could form at all in the harsh environment near a supermassive black hole," said Kastytis Zubovas of the University of Leicester in the United Kingdom, and lead author of the paper. "It's exciting because our study suggests that a huge number of them are needed to produce these flares."

Zubovas and his colleagues suggest there is a cloud around Sgr A* containing hundreds of trillions of asteroids and comets, stripped from their parent stars. Asteroids passing within about 100 million miles of the black hole, roughly the distance between the Earth and the sun, would be torn into pieces by the tidal forces from the black hole.

These fragments then would be vaporised by friction as they pass through the hot, thin gas flowing onto Sgr A*, similar to a meteor heating up and glowing as it falls through Earth's atmosphere. A flare is produced and the remains of the asteroid are swallowed eventually by the black hole.

"An asteroid's orbit can change if it ventures too close to a star or planet near Sgr A*," said co-author Sergei Nayakshin, also of the University of Leicester. "If it's thrown toward the black hole, it's doomed."

The authors estimate that it would take asteroids larger than about six miles in radius to generate the flares observed by Chandra. Meanwhile, Sgr A* also may be consuming smaller asteroids, but these would be difficult to spot because the flares they generate would be fainter.

These results agree reasonably well with estimates of how many asteroids are likely to be in this region, assuming that the number around stars near Earth is similar to the number surrounding stars near the centre of the Milky Way.

"As a reality check, we worked out that a few trillion asteroids should have been removed by the black hole over the 10-billion-year lifetime of the galaxy," said co-author Sera Markoff of the University of Amsterdam in the Netherlands. "Only a small fraction of the total would have been consumed, so the supply of asteroids would hardly be depleted."

Planets thrown into orbits too close to Sgr A* also should be disrupted by tidal forces, although this would happen much less frequently than the disruption of asteroids, because planets are not as common. Such a scenario may have been responsible for a previous X-ray brightening of Sgr A* by about a factor of a million about a century ago. While this event happened many decades before X-ray telescopes existed, Chandra and other X-ray missions have seen evidence of an X-ray "light echo" reflecting off nearby clouds, providing a measure of the brightness and timing of the flare.

"This would be a sudden end to the planet's life, a much more dramatic fate than the planets in our solar system ever will experience," Zubovas said.

Very long observations of Sgr A* will be made with Chandra later in 2012 that will give valuable new information about the frequency and brightness of flares and should help to test the model proposed here to explain them. This work could improve understanding about the formation of asteroids and planets in the harsh environment of Sgr A*.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

MEDIA CONTACTS

Trent J. Perrotto
NASA Headquarters, Washington, USA
Tel: +1 202-358-0321
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Megan Watzke
Chandra X-ray Center, Cambridge, Mass., USA
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(UK)
Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
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Kastytis Zubovas
Department of Physics and Astronomy
University of Leicester
Leicester LE1 7RH
United Kingdom
Tel: +44 (0)116 252 2084
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Dr Sergei Nayakshin
Department of Physics and Astronomy
University of Leicester
Leicester LE1 7RH
United Kingdom
Tel: +44 (0)116 252 2454
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Release and imagery on Chandra website
http://www.chandra.harvard.edu/press/12_releases/press_020812.html

http://www.chandra.harvard.edu/photo/2012/sgra/index.html

For Chandra images, multimedia and related materials, visit
http://www.nasa.gov/chandra

For an additional interactive image, podcast, and video on the finding, visit
http://chandra.si.edu

FURTHER INFORMATION

The new work is published in "Sgr A* flares: tidal disruption of asteroids and planets?"; K. Zubovas, S. Nayakshin, S. Markoff, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper can be downloaded from

http://arxiv.org/abs/1110.6872