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An Engine for Mind-Boggling Blasts

14 February 2007 (All day)
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NASA

Kaboom!
A massive star collapses to form a black hole and create a gamma ray burst. New observations suggest magnetic fields, and not jets of matter, produced the gamma rays.

Stellar explosions known as gamma ray bursts (GRBs) crank out in just a few seconds as much energy as the sun will generate over its entire 10-billion-year lifetime. Astronomers have long thought that the bursts are caused by shockwaves within ultra-high-speed jets of matter gushing out of the explosions. But new studies suggest a different source for the gamma rays: magnetic fields.

Scientists generally believe that the titanic blasts are generated when extremely large stars collapse into black holes. As a mammoth star contracts under its own immense gravity, it heats up, causing runaway nuclear reactions that send jets of matter streaming out at millions of kilometers per hour. Not all of the matter travels at the same speed, and faster matter catches up to slower-moving stuff, causing violent collisions. These shock waves heat the matter so that it glows in gamma rays, the most energetic form of radiation. At least, that has been the prevailing thinking.

Now, astrophysicist Pawan Kumar of the University of Texas in Austin and colleagues argue that the jets giving rise to GRBs are not made of matter but actually are powerful magnetic fields transporting energy away from the collapsing stars. The researchers analyzed data from 10 GRBs collected by NASA's Swift satellite and found that the sources of the bursts were located about 10 billion kilometers from the sites of the stellar collapses--about 100 times farther than expected. By the time jets of ordinary matter would have reached that distance, they could not have retained enough energy to generate gamma rays.

Instead, extremely high-energy magnetic outflows produce GRBs when they interact with surrounding atoms of gas and dust far from the black hole, the team reports in an upcoming issue of Monthly Notices of the Royal Astronomical Society. A similar process takes place on a much smaller scale on the sun, Kumar says. "Some fraction of the magnetic field energy of sunspots is deposited in charged particles and converted to [gamma] radiation," which also produces solar flares, he explains. Kumar notes that the energy in GRBs is a million-trillion times greater than the energy in solar flares.

Some researchers had speculated that the bursts might be generated magnetically, says astrophysicist Peter Meszaros of Pennsylvania State University in State College, but until now there have been only hints in one or two observations of the phenomena. So if the current finding is confirmed, he says, "it could represent a major turning point in our understanding of GRBs."

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