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Bright, Brief, But No Longer Baffling

5 October 2005 (All day)
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Death kiss. When two compact neutron stars merge into a black hole, they release a brief, energetic flash of high-energy radiation.

The incredibly bright and extremely brief flashes of energy known as short gamma ray bursts have taunted astronomers for more than 3 decades, but it looks as if researchers have nailed their origin at last. According to a series of papers in this week's Nature, these hugely powerful cosmic explosions that flood space with high-energy gamma rays are produced by merging neutron stars or black holes.

Gamma ray bursts were first detected by spy satellites in the 1960s. No one knew their source or even how distant they were until 1997, when the Italian-Dutch BeppoSAX satellite first detected a burst's afterglow, enabling researchers to determine its distance and prodigious energy output. Since then, a wealth of new observations has convinced astrophysicists that the longer bursts--which last more than 2 seconds or so--are the death cries of rapidly rotating massive stars that explode as brilliant hypernovas while their cores collapse into black holes. The gamma rays are produced by shock waves in the explosion debris. However, a large and distinct population of very short bursts, lasting 0.3 seconds on average, remained mysterious.

Now, follow-up observations of the afterglows of two short bursts on 9 May and 9 July, detected by the Swift and HETE-2 satellites, respectively, have settled the issue. The bursts, which occurred in relatively old and nearby galaxies, showed no hint of a stellar explosion and appear to have less than 1% of the power of their long-lasting cousins. These observations agree perfectly with predictions that two orbiting neutron stars--themselves compact remains of exploded giant stars--release a brief burst of gamma rays as they slowly spiral toward each other and coalesce into a black hole.

Unpublished results from three additional short gamma ray bursts support the conclusions, says Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. "The general feeling in the gamma ray burst community is that the issue has been settled," he says. Some riddles remain: Astronomers still wonder why the x-ray afterglows of short bursts appear so faint, and they still can't tell whether the merger involved two neutron stars or a neutron star and a black hole. As one team notes in its paper: "The stage is now set for detailed studies of these exotic cosmic explosions."

Related links
Animation of the merger of two neutron stars
Swift satellite
HETE-2 satellite
Background on gamma ray bursts

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