The most powerful blast ever observed in the universe was detected on Wednesday. That day, NASA鈥檚 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. 鈥淭his burst definitely has an 鈥榦h wow鈥� flavour,鈥� 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.
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Gamma rays can鈥檛 penetrate Earth鈥檚 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鈥檙e quick enough, you can study gamma-ray bursts from the ground.
That鈥檚 where NASA鈥檚 Swift satellite comes in. It detects a burst, measures its sky position, and then radioes the results to robotic telescopes on the ground 鈥� all within seconds.
With four bursts, Wednesday was the busiest day in Swift鈥檚 life so far. The second of the four bursts, GRB 080319B, occurred at 0613 GMT in the northern constellation Bootes 鈥� 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.
鈥楤urst of the year鈥�
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鈥檚 Very Large Telescope in Chile.
By studying how much the burst鈥檚 light had been stretched, or redshifted, as it travelled through the expanding universe, they put the explosion at 7.5 billion light years away. 鈥淎t first, I had expected this burst to be much closer,鈥� says Vreeswijk. 鈥淚t鈥檚 exciting to be able to see something with the naked eye halfway across the universe.鈥�
Knowing the distance, astronomers could calculate the burst鈥檚 true luminosity: 2.5 million times brighter than the most powerful supernova ever seen.
It鈥檚 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鈥檚 barrel. But, says Vreeswijk鈥檚 colleague Jens Hjorth, 鈥渋n this business, getting surprised ceases to surprise you鈥�.
Follow-up studies of GRB 080319B鈥檚 afterglow are still in progress. Says Wijers: 鈥淚t doesn鈥檛 shatter current theoretical thinking, but in terms of detailed knowledge, this will probably become the burst of the year.鈥�
