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Telescopes find signs of gentler gamma-ray bursts.



Some of the most powerful explosions in the universe could be 10 times as abundant as astronomers had assumed. That suggestion comes from two new studies indicating that many gamma-ray bursts--intense flashes of gamma-ray photons--go undetected because they don't pack quite as much punch as do the bursts that astronomers have recorded for years.

According to a leading theory, gamma-ray bursts arise when a dying star collapses to become a rotating black hole A rotating black hole (Kerr black hole or Kerr-Newman black hole) is a black hole that possesses angular momentum. It is one of four possible types of black holes that could exist in the theory of gravitation called General Relativity.  or a neutron star. The gamma rays emerge when particles jetting from a doughnut-shaped disk surrounding the collapsed star plow into surrounding space.

In the most basic version of that scenario, all gamma-ray bursts have about the same amount of energy. If so, the energy, detected at Earth would depend only on a burst's distance, the width of the jet, and how much of it is aimed toward our planet.

However, "the idea that all gamma-ray bursts spit out the same amount of gamma rays ... is simply ruled out by the new data," says Sergey Yu Sazonov of the Space Research Institute of the Russian Academy of Sciences Russian Academy of Sciences (Russian: Росси́йская Акаде́мия Нау́к,  in Moscow and the Max Planck Institute for Astrophysics The Max Planck Institute for Astrophysics is a Max Planck Institute, located in Garching, near Munich, Bavaria, Germany. It was founded as Max Planck Institute for Physics and Astrophysics  in Garching, Germany.

Sazonov and his collaborators, as well as a team led by Alicia Soderberg of the California Institute of Technology California Institute of Technology, at Pasadena, Calif.; originally for men, became coeducational in 1970; founded 1891 as Throop Polytechnic Institute; called Throop College of Technology, 1913–20.  in Pasadena, report their findings in the Aug. 5 Nature.

Both teams began their work last Dec. 3, when the European Space Agency's satellite INTEGRAL (International Gamma-Ray Astrophysics Laboratory) recorded an unusual gamma-ray burst, dubbed GRB GRB Gamma Ray Burst(er)
GRB Graduate Recruitment Bureau
GRB Grid Resource Broker
GRB Grootschalig Referentiebestand (Dutch: large scale mapping program)
GRB Gharb
 031203 for the date on which it was detected.

Just 18 seconds after INTEGRAL recorded the burst, the satellite electronically notified a network of observatories, so astronomers could pinpoint the burst's location in time to record a rapidly fading X-ray afterglow afterglow

small amounts of light emitted by a phosphor after the stimulating radiation has ceased. Seen in x-ray intensifying screens and fluoroscopic screens.
. Soderberg and her colleagues went further, recording the afterglow with radio and visible-light telescopes.

The data revealed that the gamma-ray burst had emanated from a relatively nearby galaxy--about 1.3 billion light-years away, Yet despite the burst's proximity, which presumably pre·sum·a·ble  
adj.
That can be presumed or taken for granted; reasonable as a supposition: presumable causes of the disaster.
 would enable telescopes to capture most of its gamma rays, the burst was only about one-thousandth as energetic as those originating in much more distant galaxies.

The researchers say that they can't account for the lower energy by assuming that most of the burst pointed away from Earth. For example, the afterglow of a burst spreads out over time, so the observed afterglow of a burst pointing away should rise rapidly, but the afterglow of GRB 031203 showed no such increase. "It therefore appears that this was much more likely a truly low-energy event," Sazonov says.

Astronomers already had tentative evidence that another gamma-ray burst, recorded in 1998, exploded from a nearby galaxy and was less energetic than the others that had been observed (SN: 7/10/99, p. 28). Combining the 1998 data with the new, more definitive finding, "we are now thinking that there may be many more subenergetic bursts," says Soderberg. Too faint to be detected unless they lie close to Earth, low-energy bursts could amount to as much as 10 times the number of bursts bright enough to be routinely recorded by current telescopes, she estimates.

A NASA NASA: see National Aeronautics and Space Administration.
NASA
 in full National Aeronautics and Space Administration

Independent U.S.
 satellite called Swift may unveil many of these hidden explosions, says Stan Woosley of the University of California, Santa Cruz The University of California, Santa Cruz, also known as UC Santa Cruz or UCSC, is a public, collegiate university, one of the ten campuses of the University of California. . Scheduled for launch in October, Swift will detect fainter bursts and localize lo·cal·ize  
v. lo·cal·ized, lo·cal·iz·ing, lo·cal·iz·es

v.tr.
1. To make local: decentralize and localize political authority.

2.
 them more quickly than other gamma-ray observatories do.
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Title Annotation:Explosive News
Author:Cowen, R.
Publication:Science News
Geographic Code:4EUGE
Date:Aug 7, 2004
Words:561
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