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Gamma-Ray Burst Afterglows Brighter Than Expected

The jets behind the bursts may need rethinking
GRB



Dana Berry, SkyWorks Digital

A new study casts doubt on a long-standing belief about the power behind gamma-ray bursts, the most energetic explosions in the universe.

Researchers have found that short gamma-ray bursts—those that last a couple of seconds or less—have brighter afterglows than the simple, reigning model of afterglow emission predicts.

Gamma-ray bursts (GRBs) are believed to occur when a star that has collapsed into a black hole or a neutron star whips a disk of gas and dust into a pair of powerful jets moving at nearly light speed.

Like a lighthouse in fog, these so-called relativistic jets should cause whatever gas and dust that enshrouds the GRB source to glow brightly for hours after the burst's initial flash of energy.

Long gamma-ray bursts, which flash for up to 100 seconds or longer, are believed to occur when massive stars explode as supernovae. Such dying stars have plenty of debris around them, resulting in a bright afterglow.

But short bursts form in older galaxies where such supernovae are far less common. Instead, researchers believe a more likely progenitor is a pair of neutron stars that merge into one.

Because neutron star pairs would have little gas or dust around them, researchers assumed that short bursts should have relatively dim afterglows.

That's what researchers from the Space Telescope Science Institute (STScI) in Baltimore expected to find when they combined data on 458 GRBs discovered by satellites since 2007, a painstaking chore that no one had undertaken before, says Melissa Nysewander, a former STScI astronomer and a co-author of the study submitted for publication to The Astrophysical Journal.

But when they added up the total energy emitted during each gamma-ray burst's initial flash and compared it with the afterglows half a day later, they found that bursts of the same energy glowed equally bright.

"We didn't expect that at all," Nysewander says.

If it holds up, the result could mean that researchers were wrong about the progenitors of short bursts. But Chris Fryer, a theoretical astrophysicist at Los Alamos National Laboratory in New Mexico, says it is more likely that the standard afterglow model needs revision.

That model assumes that relativistic jets store energy primarily in the form of hot matter (plasma) and less in the form of magnetic fields generated by shock waves at the front of the jets. These magnetic fields accelerate electrons, causing them to emit radiation.

Fryer says that if more of the jets' energy comes in magnetic fields, they should prompt electrons to produce more radiation, perhaps accounting for the unexpected brightness of short bursts.

He notes that the model was originally developed for active galactic nuclei—outbursts powered by supermassive black holes—so there is no reason to think it must also apply to gamma-ray bursts.

He says the simple model worked well enough when data on bursts was relatively unrefined. "As we get more and more detailed data," he says, "the simple model is breaking down."

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