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Science: Something New Brewing at the Center of a Distant Galaxy

NASA’s Swift satellite has captured an event in the cosmos like nothing ever seen before, researchers say in the 17 June issue of Science. A high-energy gamma-ray burst known as Sw1644+57, located at the center of a small, star-forming galaxy, is burning much longer and much brighter than astronomers had expected it to—and two separate teams of researchers are trying to figure out why.

Gamma-ray bursts, a flash of gamma rays associated with extremely energetic explosions in deep space, are some of the most luminous events in the universe. Given its unique characteristics, astronomers believe that Sw1644+57 could eventually unlock some of the secrets surrounding massive black holes and the turbulent processes by which galaxies form.

Andrew Levan from the University of Warwick in Coventry, United Kingdom, along with colleagues from around the world, first took note of a strong burst of gamma-ray emission coming from Sw1644+57 in the middle of a galaxy located 3.8 billion light years from Earth. The high-energy emission lasted much longer than any other gamma-ray burst on record, and it was approximately 100 times brighter than other so-called active galactic nuclei.


Artist’s impression of the initial stages of disruption of a star as it flies close to the central black hole of a distant galaxy. The star is tidally shredded, and begins to fall into the hole. | Image courtesy of University of Warwick/Mark Garlick; © Science/AAAS

“Most gamma-ray bursts that Swift captures are collapsing stars. But after its initial burst, Sw1644+57 burst three more times in just 48 hours,” said Levan. “And it’s been burning for weeks. In fact, it’s still burning right now. Sw1644+57 is actually still quite bright and still clearly visible.”


Using a network of telescopes, the researchers were able to pinpoint the location of Sw1644+57 directly at the center of a distant, star-forming galaxy. Since it was burning so brightly in that location, the researchers assumed that Sw1644+57 was a star passing too closely to a black hole, being ripped apart for its trespassing.

“But aside from the fact that it’s harboring such an amazing event, the galaxy itself is pretty unremarkable,” said Levan. “It’s a very compact galaxy, but otherwise very typical of what we see all the time.”


A color image of Sw 1644+57 created from X-ray (Chandra X-ray Observatory, blue), near- infrared (Hubble Space Telescope, green) and mid-infrared (Spitzer Space Telescope, red). The object is unique in emitting strongly at all of these wavelengths, and clearly stands out from other objects in the field. | Image courtesy of A. Levan, University of Warwick/NASA; © Science/AAAS

Sw1644+57 has displayed such unique wavelengths of X-ray and infrared light that the researchers believe they could be witnessing a new kind of tidal disruption event, which can occur when stars are devoured by powerful black holes.


Such massive black holes are known to grow by feeding on collapsing stars, and researchers believe that this process has something to do with how galaxies form. The fact that this particular event has never been glimpsed before indicates that it’s a rare occurrence. Levan and his colleagues suggest that an event like Sw1644+57 may only happen in a galaxy once every billion years or so.

“The textbook example of a tidal disruption event usually only creates a faint burst of radiation,” said Levan. “But Sw1644+57 was full of gamma-rays that ranged somewhere between 1,000 and 10,000 times brighter than the events we’ve seen before.”

Joshua Bloom from the University of California in Berkeley, California, and an international team of researchers also analyzed data from the Swift mission. They propose that Sw1644+57 could be classified as a type of small-scale blazar, or a very compact region associated with a black hole in the center of a galaxy.

Bloom and his colleagues found evidence for a powerful jet of particles associated with Sw1644+57, which implies that the event is feeding gas to the black hole and making it bigger.

Taken together, the two reports may tell astronomers more about how massive black holes and star-forming galaxies grow and evolve together.

“We don’t understand how black holes grow and how their growth is related to the growth of the galaxies that harbor them,” said Bloom. “But this is an interesting and unique event: Watching the eating habits of a big black hole.”

Brandon Bryn

16 June 2011



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