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Cosmic blasts re-evaluated
Feb. 8, 2007
Special to World Science
Scientists are
revisiting the standard explanation for the most energetic explosions in the universe, whose cause was an utter mystery until a few years ago. Artist's concept of the
Swift satellite detecting a gamma-ray burst. (Courtesy NASA)
The explosions, called long-duration gamma-ray bursts, last for seconds and occur at the edge of the observable universe.
“In just a few seconds, gamma-ray bursts emit as much energy as the Sun does in 10 billion years,” said Paul O’Brien of the University of
Leicester, U.K., one of a group of researchers presenting a new theory on the mechanism behind the bursts.
“If confirmed, this will alter our view of how these objects work.”
Scientists agree that at least some of these bursts mark the death throes of a giant star as its core collapses to form a black hole, a
thing so dense that not even light can escape its gravity.
The standard account for the bursts is that the collapsing body
spits a jet of plasma, or extremely hot gas, at nearly light speed. Different areas of the plasma have different speeds, leading to collisions within it. This in turn in turn produces intense heat and gamma rays, an extremely energetic form of light.
The new theory, to appear in an upcoming issue of the research journal
Monthly Notices of the Royal Astronomical Society, challenges that idea. In the research, Pawan Kumar of the University of Texas at Austin and colleagues argue that the energy for the gamma rays comes from a powerful magnetic field enveloping the collapsing object.
At some point, energy stored in that field is converted into electrically charged particles known as electrons, said David Burrows of Pennsylvania State University in University Park, Penn., a member of Kumar’s team. Those particles then give off part of their own energy as gamma rays. The process also leads to an afterglow of X-ray and visible light.
Using data from NASA’s Swift satellite, the team analysed 10 gamma-ray bursts recorded in 2005 and 2006.
“The gamma-ray source is located about 10 billion km [six
billion miles] from the black hole, or 100 times further than previously thought,” Kumar said. That’s far more consistent with the team’s proposed “magnetic field-dominated” outflow than with the conventional view, he added, and several other lines of evidence point the same way.
“Swift can turn and observe a gamma-ray burst with its X-ray and optical telescopes in just a few tens of seconds,” said Burrows, who is lead investigator for the satellite’s X-ray telescope. “This capability allows us to capture a snapshot of the early emission which carries information on the physical processes involved.”
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Scientists are re-evaluating the standard explanation for the most energetic explosions in the universe, whose cause was an utter mystery until a few years ago. The explosions, called long-duration gamma-ray bursts, last for seconds and occur at the edge of the observable universe. “In just a few seconds, gamma-ray bursts emit as much energy as the Sun does in 10 billion years,” said Paul O’Brien of the University of Leicester, U.K., one of a group of researchers presenting a new theory on the mechanism behind the bursts. “If confirmed, this will alter our view of how these objects work.” Scientists now agree that at least some of these bursts mark the death throes of a giant star as its core collapses to form a black hole, a dense point of matter so compact that not even light can escape. The standard account is that the black hole blasts out a jet of plasma, extremely hot gas, at nearly light speed. Different areas of the plasma have different speeds, leading to collisions within it. This in turn in turn produces intense heat and gamma rays, an extremely energetic form of light. The new theory, to appear in an upcoming issue of the research journal Monthly Notices of the Royal Astronomical Society, challenges that idea. In the research, Pawan Kumar of the University of Texas at Austin and colleagues argue that the energy for the gamma rays comes from a powerful magnetic field enveloping the collapsing object. At some point the energy stored in that field is converted into electrically charged particles known as electrons, said David Burrows of Pennsylvania State University in University Park, Penn., a member of Kumar’s team. Those particles then give off part of their own energy as gamma rays. The process also leads to an afterglow of X-ray and visible light. Using data from NASA’s Swift satellite, the team analysed 10 gamma-ray bursts recorded in 2005 and 2006. “The gamma-ray source is located about 10 billion km from the black hole, or 100 times further than previously thought,” Kumar said. That’s far more consistent with the team’s proposed “magnetic field-dominated” outflow than with the conventional view, he added, and several other lines of evidence point the same way. “Swift can turn and observe a gamma-ray burst with its X-ray and optical telescopes in just a few tens of seconds,” said Burrows, who is lead invest igator for the satellite’s X-ray telescope. “This capability allows us to capture a snapshot of the early emission which carries information on the physical processes involved.”