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Asteroids cause black hole's X-ray hiccups
Posted: 13 February 2012

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The supermassive black hole at the center of the Milky Way galaxy may be swallowing large asteroids and comets almost daily, generating bursts of X-ray light visible to NASA's Chandra X-ray Observatory, according to an international team of astronomers.

Chandra collected this mosaic of Sgr A* with 43 observations from 1999 through 2009. Red represents low-energy X-rays, green indicates medium X-rays, while the highest-energy X-rays visible to Chandra register as blue. Credit: NASA/CXC/MIT/F.K. Baganoff et al.
Scientists using data from Chandra say there is a cloud of hundreds of trillions of asteroids and comets surrounding Sagittarius A*, or Sgr A*, a region at the core of the Milky Way known to harbor a powerful black hole.

The study, which is reported in the Monthly Notices of the Royal Astronomical Society, concludes the swarm of asteroids and comets were pulled from their parent stars and thrust on trajectories toward the black hole, which then pulls apart the objects with immense tidal forces as they pass within about 100 million miles, or the distance between the Earth and the sun.

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

Astronomers have observed regular flares for several years emanating from the Sagittarius A* black hole. The flares have variable brightnesses and usually last a few hours, according to NASA.

The European Southern Observatory's Very Large Telescope in Chile has also spotted the flares in infrared light.

"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, and lead author of the report. "It's exciting because our study suggests that a huge number of them are needed to produce these flares."

Hot, thin gas surrounding Sagittarius A* vaporizes fragments from the annihilated asteroids and comets as they approach the black hole, sparking a bright flare similar to the effect of a meteor streaking through Earth's atmosphere, according to the Harvard-Smithsonian Center for Astrophysics, which controls Chandra's science and flight operations for NASA.

Artist's sketch of asteroids surrounding the Sgr A* black hole. Credit: NASA/CXC/M.Weiss
Only asteroids with a radius of at least six miles could produce flares bright enough to be observed by Chandra, the study concluded.

The scientists say planets may also be consumed by the black hole, but the demise of larger bodies is much more rare.

The center of the Milky Way is about 26,000 light years from Earth.

Hoping to shed more light on the frequency and brightness of the flares, researchers will task Chandra to look more closely at the Sgr A* star later this year. The observations could help confirm the findings of the study published this week.

Another NASA telescope, named NuSTAR, is scheduled for launch in March. NuSTAR will also study the Sgr A* black hole, but its X-ray telescope observes higher-energy X-rays and should reveal fresh views of the galactic center invisible to even advanced telescopes like Chandra.

"One of the first things we will be doing is looking at the center of the Milky Way at Sagittarius A*, which is the black hole that resides in our own galaxy," said Fiona Harrison, NuSTAR's principal investigator from the California Institute of Technology. "It's a couple million solar masses. Every once in a while it has hiccups, and that could be planets being swallowed. One of the first programs we have is to coordinate with Chandra and other observatories to watch what's happening with our own galactic black hole."

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