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A Source for Gamma-Ray Bursts

Imagine two neutron stars colliding, or even worse, a neutron star and a black hole. The release of energy would be catastrophic, and has apparently now led to the first detection in visible light from a short gamma-ray burst. Thus we’re beginning to get a handle on the most powerful explosions in the known universe, whose identity has bedeviled astronomers for thirty years.

There are actually two different kinds of gamma-ray bursts. The longer ones have been linked to the explosion of a massive star as it collapses into a black hole. It’s the short-duration bursts that have proven the greater challenge. The new work, performed at La Silla (Chile) and at the European Southern Observatory’s Very Large Telescope, used data from NASA’s HETE-2 satellite to guide the observations, and the fading source was found.

“That was the clue we were waiting for,” said Garrett Jernigan, a research physicist at the Berkeley Space Sciences Laboratory. “Bursts seem to come mainly in two varieties – the long ones, which last about 20 seconds, and the short ones, which last a few tenths of a second. It’s the short ones that have still been puzzling us.”

Neutron Stars Merging

Image: An artist’s conception of the merger of two neutron stars. Credit: European Southern Observatory.

This particular burst occurred 2400 million light years away (but see note below) in a still-forming dwarf galaxy. Astronomers are ruling out exploding stars as the cause, supporting the idea that these short-range events are the result of neutron star collisions. A second burst, detected on May 9 by the Swift satellite, pointed to a location in an elliptical galaxy 2700 million light years away. From an ESO press release:

“It is striking that the two short bursts that have finally been localised appear in quite different environments”, says Jesper Sollerman, a member of the team from Stockholm Observatory (Sweden) and Dark Cosmology Centre (Denmark). “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”, he adds.

All of which may support the collision hypothesis, in that elliptical galaxies are often rich in the kind of tight binary systems that might produce such collisions, according to the ESO materials. Centauri Dreams admits to finding this latter thought unconvincing as being based on insufficient evidence, and suspects the authors of the visible light study, which appears in the October 6 issue of Nature, would agree.

While progress in detecting sources for short-duration gamma rays is heartening, the amount we don’t know about these mysterious emmanations still dwarfs our understanding of the processes behind them. Centauri Dreams prefers this statement from one of the study’s authors: “Our observations do not prove the coalescence model, but we surely have found a lady with a smoking gun next to a dead body,” said Shri Kulkarni of the California Institute of Technology.

Four papers report on gamma-ray observations in the September 5 issue of Nature, all of which are abstracted here.

Sources: European Southern Observatory; California Institute of Technology; University of California at Berkeley. That press releases do not always agree would surprise no one, but the extreme difference in distance cited among these sources — 1 billion light years vs. 2.4 billion for the first gamma-ray source — does point out the dangers of fast reporting on still evolving stories.