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Astronomers spot weirdo gamma ray burst

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NASA boffins have spotted a new type of explosion which they are calling a hybrid gamma ray burst. The explosion was detected by the Swift satellite in June this year (2006).

Scientists are only just beginning to understand what happens in a gamma ray burst, so finding a new type is not altogether surprising. It has a baffling mix of properties, sharing some characteristics with both known types of gamma ray bursts (GRBs). It also has some features that are as yet unexplained.

"We have lots of data on this event, have dedicated lots of observation time, and we just can't figure out what exploded," said Neil Gehrels of NASA Goddard Space Flight Centre, lead author on one of four reports appearing in this week's edition of the journal Nature. "All the data seems to point to a new but perhaps not so uncommon kind of cosmic explosion."

Like other GRBs, scientists think this explosion is associated either with the collapse of a massive star or with the collision of two stars, most likely signalling the birth of a new black hole. The bursts, the most powerful explosions in the universe since the big bang occurred, are normally very short lived - lasting from less than a second to just a few minutes.

The hybrid burst lasted for 102 seconds, NASA reports, classifying it solidly as a long-lived burst. Normally after a long burst, astronomers expect to see evidence of a supernova, but there has been none.

"This was close enough to detect a supernova if it existed," said Avishay Gal-Yam of Caltech. "Even Hubble didn't see anything."

The explosion also originated in a galaxy with a low rate of star formation and few stars that would be massive enough to go supernova, adding to the mystery of what exactly caused the burst.

Additionally, NASA reports that the burst has some characteristics that suggest it was the result of a merger of two stars. But no theoretical models exist that can account for the sustained release of energy. The scientists say they are in brand new territory, with no theory to guide them.

Data collected on bursts from the 1990s suggest that hybrid bursts could have been seen before, but there are no follow up observations that can be made. Researchers instead have to wait until another hybrid burst occurs before they can gather any more data.

One of the reasons GRBs are important is that they tell us a lot about the early universe. The explosions are so bright they could be detected as far back as the earliest five per cent of the universe's lifetime.

As a star must either collapse or collide with another star to produce one, so their presence is conclusive evidence of star formation. This gives us an insight into when stars began forming, and what the universe must have been like billions of years ago. ®

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