Universe's most powerful blast visible to the naked eye

The most powerful blast ever observed in the universe was detected on Wednesday. That day, NASA's Swift telescope found a record four gamma-ray bursts.

If you knew exactly when and where to look, you could have seen the bright burst with the naked eye, despite its enormous distance of 7.5 billion light years. "This burst definitely has an 'oh wow' flavour," says astrophysicist Ralph Wijers of the University of Amsterdam in the Netherlands.

Gamma-ray bursts are brief but extremely powerful flashes of high-energy radiation. Theorists think they signal the violent death of very massive, rapidly rotating stars.

Gamma rays can't penetrate Earth's atmosphere, so they can only be observed by space telescopes. But many bursts also produce lower-energy X-rays, radio waves and even visible light. So if you're quick enough, you can study gamma-ray bursts from the ground.

That's where NASA's Swift satellite comes in. It detects a burst, measures its sky position, and then radioes the results to robotic telescopes on the ground - all within seconds.

With four bursts, Wednesday was the busiest day in Swift's life so far. The second of the four bursts, GRB 080319B, occurred at 0613 GMT in the northern constellation Bootes - well-placed for follow-up observations with telescopes in the US.

One of these robotic telescopes, called RAPTOR, was already looking in that part of the sky. It witnessed the quick rise and fall of an optical flash. About 30 to 40 seconds after the Swift detection, the burst peaked at naked-eye visibility, making this the only gamma-ray burst so far that could have been seen without a telescope.

'Burst of the year'

But it took a few more hours to determine how powerful the burst really was. Paul Vreeswijk of the Dark Cosmology Center in Copenhagen, Denmark, led a team that measured the distance to the burst using the European Southern Observatory's Very Large Telescope in Chile.

By studying how much the burst's light had been stretched, or redshifted, as it travelled through the expanding universe, they put the explosion at 7.5 billion light years away. "At first, I had expected this burst to be much closer," says Vreeswijk. "It's exciting to be able to see something with the naked eye halfway across the universe."

Knowing the distance, astronomers could calculate the burst's true luminosity: 2.5 million times brighter than the most powerful supernova ever seen.

It's unclear what exactly caused this incredible brightness, but most theorists think that gamma-ray bursts produce two narrow jets of matter and energy, so we may have been lucky to look right into the cannon's barrel. But, says Vreeswijk's colleague Jens Hjorth, "in this business, getting surprised ceases to surprise you".

Follow-up studies of GRB 080319B's afterglow are still in progress. Says Wijers: "It doesn't shatter current theoretical thinking, but in terms of detailed knowledge, this will probably become the burst of the year."

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