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Long-lasting gamma ray bursts were death throes of a supergiant - study

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Scientists studying one of the biggest cosmic explosions ever recorded have theorised that the enormous bang, from which super long-lasting gamma ray bursts were emitted, was caused by the death of a massive star up to 1,000 times bigger than the Sun.

A team at the University of Warwick studied a mysterious explosion recorded on Christmas Day 2010, which sent a burst of gamma rays shooting across the universe.

Lethal gamma rays are produced during the deaths of massive stars, but are usually only emitted for up to about a minute. The Christmas Day recording went on for several hours, suggesting this was a bang bigger than humans had ever witnessed.

Dr Andrew Levan, from the University of Warwick, started out studying the Xmas explosion and then found several similar examples.

He said: “These events are amongst the biggest explosions in nature, yet we’re only just beginning to find them.

“It really shows us that the Universe is a much more violent and varied place than we’d imagined.

“Previously we’ve found lots of gamma-ray events with short durations, but in the past couple of years we’ve started to see the full picture.”

He found that the gamma ray burst came from an explosion in a galaxy almost half-way to the edge of the observable universe, or 7 billion light years from earth.

After deducing the location of the star using data from the Gemini Telescope in Hawaii, Levan’s team developed a new theory that the explosion was caused by a supergiant. These are huge stars which eventually evolve to become the “biggest and brightest stars in the universe” with a radius of up to 1 billion miles, before exploding in a dramatic supernova.

When these super massive stars go pop, they emit a long stream of gamma rays which last for much longer than bursts produced by small and dense stars.

In small stars, the gamma rays created in the explosion take a short time to reach the edge of the star, where they are propelled into space. With huge stars, the explosion takes much longer to travel through the star, meaning the gamma ray burst is longer.

The most common type of gamma-ray burst is thought to be caused when a Wolf-Rayet star in the final phase of its evolution collapses into a black hole at its core.

The black hole draws matter inside, due to its huge gravitational tug, but some of the energy escapes and is focused into two jets of materials, which blast out in two directions. Travelling at the speed of light, allowing them to surge away from the black hole’s gravity, these bursts surge into space, bringing high concentrations of gamma rays with them.

However, a gamma-ray burst in a bigger star the size of a supergiant needs to power through a larger reservoir of material, hence its longer duration.

Nial Tanvir, a professor at the University of Leicester and second author of the study, said: “We believe that powering the explosion is a newly formed black hole in the heart of the star.

“Predicting the detailed behaviour of matter falling into a black hole in these circumstances turns out to be very difficult, and from a theoretical point of view we didn’t initially expect explosions at all.

“The amazing thing is that nature seems to have found ways of blowing up a wide range of stars in the most dramatic and violent way.”

This research was scheduled be presented at the GRB 2013 Symposium in Nashville, Tennessee yesterday.

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