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NASA Satellite Data Shaping Gamma Ray Burst Theories
By ROBERT ROY BRITT
Special to Space News
posted: 11:20 am ET, 12 June 2003
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NASHVILLE — For decades researchers have searched for the cause of the incredible explosions from deep space called gamma ray bursts. Investigations into two such recent events have ruled out one theoretical model and provide support for another, while also hinting at the mechanism behind the bursts.
Gamma ray bursts (GRBs) shine briefly with the intensity of one million trillion suns. One or two a day go off, scattered throughout the cosmos.
Scientists believe gamma ray bursts are caused by supernovae, explosive collapses of the most massive stars, which end up forming black holes or dense neutron stars. A beam of energy shoots out along the axis of the star’s rotation, theory holds. If that beam is aimed at Earth then a gamma ray burst can be recorded.
But the specific mechanism for triggering the immense release of energy is not known.
GRBs are elusive. Some last less than a second and though intense, they typically originate billions of light-years away, so the amount of energy that reaches our solar system is relatively limited. The two events in question, however, were bright enough and recorded quickly enough to be well-studied.
The closest known gamma ray burst — 2.6 billion light-years away — was spotted by NASA’s High-Energy Transient Explorer satellite March 29.
Subsequent observations of GRB 030329, as it is called, are the most detailed ever made of a gamma ray burst and its so-called afterglow — radiation that continues to pour out in other wavelengths, from radio waves to visible light and X-rays.
Data from the National Science Foundation’s Very Long Baseline Array and other telescopes showed that matter streamed smoothly from the collapsing star many days after the initial eruption at a rate of about 98 percent of the speed of light, said Dale Frail of the National Radio Astronomy Observatory, which operates telescopes throughout the Western Hemisphere.
The observations of the March 29 gamma ray burst supported the leading model for such phenomena, which is known as the fireball model, which supposes a continuous stream of matter and energy racing outward from the star. The data from that event also refuted another theory, known as the cannonball model, which held that gamma ray bursts were Earth-sized blobs of material that were shot out of a supernovae.
"We’re not ready to concede victory," Frail joked, adding that more data has been collected and needs to be analyzed. "But I think we’ve clearly ruled out the alternative [cannonball] model."
After March 29, the cannonball theory’s creators claimed to predict subsequent phenomena that should be observable, and they were right. Frail said, however, that the fireball model could have been used to make similar prediction. Frail said the cannonball was a viable model until now.
Other researchers agreed with Frail’s assessment.
Steven Boggs of the University of California, Berkeley, led observations of an earlier gamma ray burst that shed additional light on gamma ray bursts.
A Dec. 6 event monitored by NASA’s Reuven Ramaty High Energy Solar Spectroscopic Imager, detected polarization in the radiation of the beamed jet ejected by the burst.
Photons of all light are polarized, either sideways or up-and-down, said Boggs. His team looked at the average polarization of all the radiation — whether it tended to be more of one type or the other. The light was polarized about 80 percent in one direction.
Boggs said the polarization of the light provided clues to the geometry of the source that generated it. Large and strong magnetic field lines appear to have been wound up, much like strings on a top, then were spat out as the star collapsed, "probably beamed in our direction," Boggs said.
Both findings were presented here May 28 at a meeting of the American Astronomical Society.
Chryssa Kouveliotou, of NASA’s Marshall Space Flight Center, a 30-year veteran of gamma ray burst research, but was not involved directly in the new studies, called Boggs’ results a milestone.
Frail’s results "show that the fireball model is alive and well and that all of the competing models are dead," Kouveliotou said.
She said it’s still unclear how much total energy a gamma ray burst puts out. And there is another, larger question that remains: "What is the beast in the center?"
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