Gamma-Ray Burst mystery solved:
exploding stars the culprit
Australian telescopes have helped provide the clinching evidence that gamma-ray bursts the biggest bangs in the Universe are produced when massive stars explode and their cores collapse to form black holes.
An international team of astronomers led by Professor Shri Kulkarni of the California Institute of Technology (Caltech) got their proof from a gamma-ray burst that occurred last November, GRB 011121.
Gamma-ray bursts are enormous blasts of gamma rays, often accompanied by an 'afterglow' of light, X-rays and radio waves.
They were discovered in 1967. Their cause has been unknown, although evidence has been building since 1997 that massive stars are the culprits.
Careful sleuthing with the Hubble Space Telescope, CSIRO's Australia Telescope Compact Array radio telescope, the Anglo-Australian Telescope and telescopes in Chile showed that gamma-ray burst GRB 011121 had indeed been accompanied by the explosion of a massive star a supernova.
The Hubble Space Telescope picked up the tell-tale light 'signature' of a supernova while the ground-based telescopes showed that the explosion had taken place in a cocoon of matter shed by the star before its demise.
"Once it became clear that we had seen not only the supernova but also the cocoon I was very happy; I couldn't sleep for days," says team member Paul Price, a PhD student at the Australian National University.
"From the radio observations you can get evidence for the structure of the material around the star, the mass-loss history of the star," says CSIRO's Dr Mark Wieringa, who monitored the object with CSIRO's Australia Telescope.
Dr Stuart Ryder of the Anglo-Australian Telescope made infrared observations with the Anglo-Australian Telescope's new IRIS2 instrument. "Infrared radiation penetrates dust better than does light," Dr Ryder explains.
"By comparing the infrared and optical observations we were able to determine the amount of dust between us and the gamma-ray burst something that hasn't been done before." The dust data confirmed that the gamma-ray burst occurred inside matter shed by the parent star.
The idea that gamma-ray bursts and supernovae are linked goes back more than a decade. But it got a boost in 1998 when Australian telescopes the Australia Telescope, the Anglo-Australian Telescope and telescopes of the ANU saw for the first time a supernova that appeared to coincide with a gamma-ray burst. "It was this supernova [SN1998bw] that really pushed people to think about the link between gamma-ray bursts and supernovae," says Dr Dale Frail of the National Radio Astronomy Observatory, a member of the team studying GRB 011121.
GRB 011121 was detected in the southern sky on 11 November 2001 by the Italian-Dutch satellite BeppoSAX. Its position was further pinned down by a network of satellites. Optical telescopes in Chile identified the glowing embers of the gamma-ray burst and determined that the burst was located in a relatively nearby galaxy, only five billion light-years from Earth.
Gamma-ray bursts can be seen for vast distances across the Universe. If they are caused by the core collapse by massive stars it may be possible to use them to trace star formation in the early universe, perhaps even back to the first generation of stars.
But Professor Kulkarni cautions that not all is yet known about gamma-ray bursts. It may be that some bursts are produced by other exotic phenomena, such as two colliding neutron stars or a neutron star colliding with a black hole.
Research papers on GRB 011121 will appear in the 10 June 2002 issue
of The Astrophysical Journal Letters (vol 572). They are also available
online via http://xxx.lanl.gov/abs/astro-ph/0203391 and http://xxx.lanl.gov/abs/astro-ph/0203467
Dr Stuart Ryder, Anglo-Australian Observatory
Dr Mark Wieringa, CSIRO Australia Telescope National Facility +61-2-6790-4040
Paul Price (currently visiting Caltech)
Dr Dale Frail, US National Radio Astronomy Observatory
Prof. Brian Boyle, Director, Anglo-Australian Observatory
Prof. Shri Kulkarni, Caltech Astronomy Department
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