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UK boffins uncover more secrets of the gamma ray burst

Optical afterglow shedding some light

New observations of one of the largest and most violent explosions in the universe have provided valuable clues for intergalactic sleuths seeking to understand the physics of Gamma Ray Bursts.

Scientists at the Liverpool John Moores University were able to begin observing the burst just 203 seconds after it had begun, thanks to the early warning given by NASA's Swift satellite.

The scientists from Liverpool John Moores University and their colleagues in the UK, Italy, France and Slovenia used the Liverpool Telescope on the island of La Palma and its new polarimeter, RINGO, to perform the measurement.

Gamma Ray Bursts are very short lived, but incredibly powerful explosions, so bright that we can see them as far back as the earliest five percent of the universe's life time. It is thought that a star must collapse, or two stars must collide to produce one, so their presence is seen as good evidence of star formation. This is important because it gives us an idea of when stars began forming, and what the universe must have been like, billions of years ago.

But because they are so brief - lasting from a few seconds to maybe a few minutes - very little in known about them. The launch of the Swift satellite is changing all that because it sets in motion a cascade of observations in space and on the ground the moment it detects a blast.

This latest explosion has revealed a huge amount of detail about the polarisation of the "optical afterglow", the burst of light emitted in the blast that is thought to be caused by ejected material impacting the gas surrounding the dying star.

Until now, the composition of the ejected material has remained a mystery and, in particular the importance of magnetic fields has been hotly debated by GRB scientists. Either way, the early optical glow contains important clues for both these areas of research.

Principal author of the Science paper, Dr Carole Mundell of the Astrophysics Research Institute, Liverpool John Moores University, explains:

"Our new measurements, made shortly after the Gamma Ray Burst, show that the level of polarisation in the afterglow is very low. Combined with our knowledge of how the light from this explosion faded, this rules-out the presence of strong magnetic fields in the emitting material flowing out from the explosion - a key element of some theories of GRBs."

However much scientists have learned from this set of data, gamma ray bursts remain hugely mysterious events. Lord Martin Rees, Astronomer Royal and president of the Royal Society said that science was "still flummoxed: by the underlying trigger of the explosions, and why they sometimes emit brief flashes of light.

"Theorists have a lot of tentative ideas, and these observations narrow down the range of options," he added.

The research is publised in the March 15 edition of Science

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