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05/16/2002 07:00:00

Gamma-ray bursts are caused by explosive death of massive stars, new study reveals

In two papers appearing in an upcoming issue of the Astrophysical Journal, an international team of astrophysicists led by Shri Kulkarni of the California Institute of Technology reveals that new data show that supernovae are the source of gamma-ray bursts.

The new information was obtained from a gamma-ray burst that was detected in November and studied by the Hubble Space Telescope, the Australia Telescope Compact Array, the Anglo-Australian Telescope, and optical telescopes in Chile.

For the last few years astronomers have been chasing clues linking the mysterious gamma-ray bursts to their favored suspect: massive stars. Previous observations hinted at debris from an exploding star, but the observations were inconclusive.

Careful observations of gamma-ray burst GRB 011121 have uncovered remnants of the exploded star, whose signature was buried in the bright, fading embers. Now, for the first time, two compelling tell-tale signatures of the massive star were observed.

As explained by Kulkarni, who is the McArthur Professor of Astronomy and Planetary Sciences at Caltech and the head of the international team that made this discovery, "With these observations we have tied this gamma-ray burst to an exploding star. I am absolutely delighted that nature provided us with such a clean answer."

At the core of the observations, the data show that a supernova accompanied the burst. Supernovae are a natural consequence of exploding stars and difficult to make by other means. Joshua Bloom, Caltech graduate student and lead author on the supernova paper, said. "It is not often that a graduate student gets the chance to make a major discovery. I am very fortunate to be involved in this one."

The astronomers were also able to deduce that the explosion took place in a cocoon of gas fed by a "wind" of matter emanating from the progenitor star. Paul Price, graduate student at the Australian National University and a lead author on the second paper, was "intensely excited. Once it became clear that we had not only seen the supernova but also the cocoon I was very happy; I couldn't sleep for days."

The gamma-ray burst in question was detected on November 21, 2001, by the Italian-Dutch Satellite BeppoSAX in the southern-sky constellation of Chamaeleon. The position was quickly refined by a network of satellites. Astronomers from Poland and Chile, as well as another U.S. team from Harvard, used optical telescopes in Chile to rapidly identify the ``afterglow,'' or glowing embers, of the gamma-ray burst and determined that the galaxy in which the burst was located was quite near - a paltry five billion light-years from Earth.

The sensitive optical and infrared observations were in part possible because of the relatively small distance to the burst. Given the proximity, the Caltech team decided to dedicate a large portion of their allocated Hubble Space Telescope time toward observing any possible supernova component. Kulkarni says of the decision, "We simply went for broke because of the potential payoff."

Kulkarni believes that this is just the beginning of a new era in our understanding of the death of massive stars. The stars die by collapsing, and the collapse both fuels the explosion and leaves a stellar residue of neutron stars and black holes. Indeed, theorists have long speculated that gamma-ray bursts are the birth cry of spinning black holes. New facilities such as the Chandra X-Ray Observatory, and future facilities such as gravitational-wave observatories and neutrino telescopes, will allow astronomers to investigate the dramatic collapse process.

Kulkarni cautions, however, that all is still not known about gamma-ray bursts. It may be that other exotic phenomena, such as two colliding neutron stars, or a neutron star colliding with a black hole, produce some of the events that we see. "Despite extensive efforts, until now we have not seen clear signatures for a cocoon in dozens of other gamma-ray bursts, and there have been only hints of a supernova in a few other bursts," Bloom says.

Price adds, "It means there will be lots more to do in the future. I have a secure thesis now!"

In addition to Kulkarni, Bloom, and Price, members of the team reporting the results are Caltech professors S. George Djorgovski and Fiona Harrison and postdoctoral fellows and scholars Daniel Reichart, Derek Fox, Titus Galama, and Re'em Sari. Edo Berger and Sara Yost are Caltech graduate students also on the team, as are Dale Frail from the National Radio Astronomy Observatory, and many other international collaborators. Separately, P. M. Garnavich of the University of Notre Dame and his collaborators have reached similar conclusions with data taken from the Magellan telescope in Chile.

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Written by Robert Tindol