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Astronomers probe mystery of short GRBs

That's Gamma Ray Bursts, to you

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Astronomers have for the first time observed the visible afterglow of an elusive short-lived gamma ray burst (GRB).

Long-lived bursts have been seen in the optical spectrum before, but the short bursts are so short that no telescope has ever been able to respond to one quickly enough to catch it in the act. By definition, the short bursts last less than two seconds, but they are often far shorter.

The explosion, lasting just 70 milliseconds, was spotted by the NASA HETE-2 satellite on 9 July this year. The data from the satellite was used to determine the location of the burst in the sky.

Within 33 hours an international team of astronomers led by Danish astronomer Jens Hjorth, had more powerful ground-based telescopes (the Danish 1.5m telescope at ESO La Silla) recording the fading optical glow. The burst in July was detected about 11,000 light years out from the centre of a young, star forming dwarf galaxy, itself some 2.4bn light-years from Earth.

Observations of the area over the following 20 days led the astronomers to rule out a hypernova event, usually responsible for the long burst GRBs, as the source of the burst.

This, the researchers say, lent more credence to the idea that some other mechanism, possibly the collision of two neutron stars, is behind the short bursts.

The Swift satellite, specifically designed to improve detection of and response times to GRBs, had previously pinpointed the location of an even shorter lived burst. Although no optical afterglow was seen in this case, the fact that it was found in an elliptical galaxy also supports the theory that short bursts are the result of the merger of two small, but dense bodies.

Elliptical galaxies typically have few large stars that could go hypernova, but are rich in binary systems, with pairs of compact stars closely orbiting each other.

Hjorth and his colleagues warn against jumping too quickly to definitive conclusions, however.

Jesper Sollerman, a member of the team responsible for the discovery, notes: "It is striking that the two short bursts that have finally been localised appear in quite different environments. The most important aspect of these discoveries is probably that we have finally shown that the short bursts are indeed cosmic explosions from far away in the Universe." ®

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