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Star eaten by black hole
Posted: 16 June 2011

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The moment a star strayed too close to a hungry black hole was captured by the Swift satellite, which was blasted in the high energy death cry of the star as it was ripped apart.

The gamma-ray flare, known as Sw 1644+57, emanated from a galaxy 3.8 billion light years away in the constellation Draco, and was first spotted on 28 March. Surprisingly, the event is still going on, telling the astronomers that this was no ordinary gamma-ray burst that might normally be associated with the collapse of a massive star at the end of its life.

Artist’s impression of the star about to be ripped apart. Image: University of Warwick / Mark A. Garlick.

"This is truly different from any explosive event we have seen before. This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than two and a half months later," says Joshua Bloom of the University of California, Berkeley, lead author of one of two papers published in the journal Science this week on the flaring event. "That's because as the black hole rips the star apart, the mass swirls around like water going down a drain, and this swirling process releases a lot of energy."

The stellar victim was likely a similar mass to the Sun, and was shredded by a black hole a million times more massive. The astronomers also saw bright flares as further chunks of the star were consumed by the black hole. It was pure chance that the radiation was pointed straight at the Earth.

Artist’s impression of formation of two jets of energy at Swift 1644+57. Image: University of Warwick / Mark A. Garlick.

“Despite the power of this the cataclysmic event we still only happen to see this event because our Solar System happened to be looking right down the barrel of this jet of energy,” says Andrew Levan of the University of Warwick, lead author of the second paper.

Although tidal disruptions of stars have been seen before at X-ray, ultraviolet and optical wavelengths, this is the first time that gamma-ray energies have been observed, and Bloom says that this is probably a once in a 100 million year event in any given galaxy. "I would be surprised if we saw another one of these anywhere in the sky in the next decade," he says. "We think this event was detected around the time it was as bright as it will ever be, and if it's really a star being ripped apart by a massive black hole, we predict that it will never happen again in this galaxy."

The gamma-ray flare likely began a few days before the Swift satellite picked it up, and may take up to a year to fade away. Once Swift had made the detection, further observations were made with the Hubble Space Telescope, Chandra X-ray Observatory, the Gemini and Keck telescopes in Hawaii, and the United Kingdom Infrared Telescope.