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Gamma Ray Needles May Pierce Space

12 September 2003 (All day)
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Piercing. Gamma rays may stream from collapsing stars in sharply focused beams no more than 1 angular degree wide.

Gamma ray bursts (GRBs) and other cosmic explosions called x-ray flashes are the same phenomenon in different guises. That's the prediction of a disputed new model in which GRBs--the most powerful explosions in the universe--aim their energy like a rifle shot, whereas x-ray flashes emit broader fans of energy. If true, up to 100,000 GRBs could blow up in the universe for each one that astronomers see.

Satellites spot GRBs as flares of the most energetic radiation known. Astronomers reached a consensus about their origins earlier this year by finding the signature of an exploded star--a supernova--at the site of a nearby GRB (ScienceNOW, 14 April). Theorists now concur that massive stars must spew fantastic jets of energy into space when their cores collapse into black holes, but they disagree about what those jets look like. Clues lie in comparing GRBs to their seemingly less-energetic cousins, x-ray-rich GRBs and x-ray flashes, whose sources are more mysterious.

New research suggests all three types arise from supernovas that focus their fury in jets with drastically different shapes. Astrophysicist Donald Lamb of the University of Chicago and colleagues examined the energies of GRBs and their x-ray kin recorded by the High-Energy Transient Explorer-2 (HETE-2) satellite and earlier orbiters. The team saw a tight correlation between the total energy observed for each blast and the peak wavelength of the measured radiation. The bursts that appeared to be the most powerful churned out most of their energy in intense, short-wavelength gamma rays, while the weakest ones had peak energies at longer wavelength x-rays.

This striking pattern implies that the three types of explosions all have the same overall energy. If that's the case, says Lamb, an x-ray flash must spew radiation in nearly all directions, dimming its impact for a distant observer. However, a GRB would channel its outburst into a needlelike cone perhaps 1 angular degree wide, which we see as a far brighter flare. "Their jets are so tiny that we only see them if we are in the boresight," says Lamb, noting that we would detect just one of every 10,000 or 100,000 such blasts. Lamb spoke on 11 September at the GRB 2003 Symposium in Santa Fe, New Mexico.

His scenario earned a mixed reception. Astronomer Dale Frail of the National Radio Astronomy Observatory in Socorro, New Mexico, says the unified model "is so simple and elegant, you want it to be true." But astrophysicist Shrinivas Kulkarni of the California Institute of Technology in Pasadena objects to the "incredibly tiny" 1-degree beams. Rather, his group's research points to a range of gamma ray and x-ray energies emerging together in wider cones.

Related sites
GRB 2003 Symposium, with links to abstracts
HETE-2 satellite
A NASA history of GRB research

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