Image
© Mark Garlick, University of Warwick handout
Artist's impression of the aftermath of star being consumed by a massive black hole in a galaxy 3.8 billion light years away. It blasted jets of energy from the black hole, one of which pointed directly at our own galaxy, enabling scientists to study and reconstruct the cosmic drama.
In late March, NASA's Swift satellite picked up a blast of gamma rays screaming past Earth.

Astronomers rushed to take a closer look, using powerful telescopes from Hawaii to the Canary Islands to check out the high-energy jet coming from a distant galaxy in the constellation Draco.

They initially speculated a collapsing star created the blast. Now they report that it appears a star the size of the sun was shredded by a massive black hole. Its "death rattle" was a high-energy flash or jet pointed straight at the Earth.

"This is a very strange one," says Nicholas Law, at the University of Toronto's Dunlap Institute for Astronomy and Astrophysics, a member of the international team that describes the cosmic drama in two reports published Thursday by the journal Science.

NASA's Swift satellite is designed to pick up brief, intense flashes of gamma radiation. The blasts can come from any direction in the sky and typically last from a few milliseconds to minutes.

After detecting the burst March 28, Swift alerted ground dates and a bulletin went out to astronomers, who focused everything from the Gemini North telescope in Hawaii to the Hubble Space Telescope on the unusually bright and long-lasting emissions from a galaxy nearly 4 billion light years away.

"With something like this you really want get on it as soon as you possibly can," says Law.

Bad weather marred the view from the Hawaiian telescope but the astronomers soon had several telescopes focused on the outburst that they describe as "unlike any previously observed."

"This is truly different from any explosive event we have seen before," Joshua Bloom, of the University of California Berkeley, says in a release. Bloom is lead author of one of the Science reports.

It was one of the biggest and brightest bangs yet recorded, the scientists say, and the evidence indicates it came from a massive black hole at the centre of the distant galaxy. They say the high-energy X-rays and gamma rays persisted at an extremely bright level for weeks, with flares when chunks of the star fell into the black hole.

"This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than 2 1/2 months later," says Bloom. "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."

It's estimated that about 10 per cent of the star's mass turned into energy generating the beam or jet of X-rays and gamma rays. Earth just happened to be in its path. "We're looking down the barrel," says Bloom.

Law, at the University of Toronto, is part of an international project called the Palomar Transient Factory, which has been doing a systematic survey of the night sky for two years, looking for everything from planets to quasars.

He and his colleagues went back through their records and found they had looked at the galaxy 77 times before the explosion.

"It was a very faint blob that was doing absolutely nothing as far as we could tell," Law said. There was no evidence of X-ray or gamma ray emissions before the blast in March, leading the scientists to conclude it was a one-off event caused when a star wandered too close to the black hole.

Although the star died about 3.8 billion years ago, the jet and energy it produced is just now passing by Earth. By studying its signature, the scientists can reconstruct what happened.

There is also a black hole at the centre of our Milky Way galaxy, and Bloom says it "occasionally burbles or hiccups as it swallows a little bit of gas."

But Law notes that our sun and solar system are "a very long, long way from the galactic centre. I think it is safe to say we're never going to fall into the black hole at the centre of our galaxy."

But he says it is fascinating to watch a star being shredded from afar. "It is a lot of fun to see what the universe is capable of," says Law.