New gamma-ray bursts

Print edition : May 17, 2013

An artist's impression of the stars creating GRBs. The blue star in the background is the progenitor fo a standard long-duration GRB. The star in the foreground is the suggested progenitor of an ultra-long GRB. Photo: Mark A. Garlick, used with permission by the University of Warwick

COSMIC explosions create powerful blasts of high-energy gamma rays, known as gamma-ray bursts (GRBs). While GRBs of the most common type are over in about a minute, a team of astronomers has pinpointed a new, exceptionally powerful type that can last for several hours. The first example was found by astronomers on Christmas day in 2010, but it lacked a measurement of distance and so remained shrouded in mystery, with two competing theories put forward for its origin.

A new study by the team, led by Andrew Levan at the University of Warwick, U.K., finds several more examples of these unusual GRBs and shows that the Christmas GRB took place in a galaxy much farther away than the two theories suggested.Using data from the Gemini Observatory in Hawaii, the team calculated that this ultra-long GRB had a redshift of 0.847, which gives it a location of about halfway to the edge of the observable universe. On the basis of this data, Levan’s team developed a new theory to explain how it occurred. It suggests this kind of burst is caused by a supergiant star 20 times more massive than the sun, which evolves to become among the biggest and brightest stars in the universe with a radius of up to one billion kilometres, 1,000 times that of the sun. The scientists believe the Christmas GRB and two other similar bursts are simply due to the sheer size of the supergiants exploding in a supernova.

The more common GRBs are thought to be caused when a Wolf-Rayet star collapses into a black hole at its own core. Matter is drawn into the black hole, but some of its energy escapes and is focussed into a jet of material which blasts out in two directions forming copious gamma rays in the process. Such stars are thought to be relatively small and dense, and the explosion that destroys them punches through them in a matter of seconds. In the case of these new ultra-long bursts, the explosion takes much longer to propagate through the star, and so the GRB lasts for a much longer time.

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