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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.

The ex­plo­sions, called long-duration gam­ma-ray bursts, last for sec­onds and oc­cur at the edge of the ob­serv­a­ble uni­verse.

Artist's con­cept of the Swift sat­el­lite de­tect­ing a gamma-ray burst. (Cour­te­sy NASA)


“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.”