 Illustration: Gamma-Ray Burst
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Nov. 17, 2003 — Mysterious gamma-ray bursts are indeed formed as a result of supernovae collapsing into a black hole, new research has confirmed.
Researchers from Australia, the United States and the U.K. — including Paul Price of the Mount Stromlo Observatory in Canberra — reported their findings in last Thursday's issue of the journal Nature.
Gamma-ray bursts are the most violent and powerful explosions in the universe. They were first picked up in the 1960s by the U.S. military who thought they were Russian nuclear explosions. Since then scientists have discovered they come from deep space — billions of light years away. Yet what exactly causes them has been a mystery.
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According to Paul Francis, a senior lecturer at the Australian National University and astrophysicist at the Mount Stromlo Observatory, there have recently been two main competing explanations of what causes gamma-ray bursts. He said the paper by Price and the link with supernovae, otherwise known as collapsing stars, offers an "important clue" as to which may be right.
While one theory has been that gamma-ray bursts are formed by colliding black holes or neutron stars, Price has been testing the other theory — that gamma-ray bursts are formed as a result of supernovae collapsing into a black hole. Earlier this year, Price and colleagues discovered the best evidence to support this theory — linking for the first time a nearby gamma-ray burst with a nearby supernova.
"For the first time there was compelling evidence that at least one gamma-ray burst was connected to a supernova," Francis told ABC Science Online.
But there was one gap in the theory. The gamma-ray bursts Price was looking at were nearby, while most gamma-ray bursts were 100 times further away — and 100 times more powerful. Scientists were worried Price's findings could not be generalized, Francis said. Perhaps the gamma-ray bursts he was studying were 'wimps', not representative of the majority.
However, now Price and colleagues have found that the nearby gamma-ray bursts are in fact just as powerful as those which are far away: "What this paper shows is that their findings are in fact generalizable," Frances said.
Apart from gamma-rays, scientists know that gamma-ray bursts also give off light and radio waves. Price and colleagues have found that gamma-ray bursts occur in two phases.
First, there is a very short and tightly focused explosion that produces the actual gamma-ray signal. Then there is a cumulatively more energetic outburst lasting a few days that produces an 'afterglow' of light and radio waves.
In the same way that a 5W laser beam can appear more powerful than a 5W light bulb, the short, sharp focused burst of gamma-rays have appeared to be the most energetic part of a gamma-ray burst — hence the name. However, Price and colleagues have found that this brief burst of energy actually only makes up around 1 percent of the total energy of the gamma-ray burst. When this is considered, the total amount of energy in the nearby gamma-ray burst — including radio, light and gamma-rays — is similar to the amount of energy given off by far-away gamma-ray bursts.
"What they've found is that this nearby gamma-ray burst is a very typical gamma-ray burst if you look at 99 percent of the energy given off," Francis said.
"This is important work because these things have been a mystery for 30 years," he said. "Probably all the gamma-ray bursts are supernovae."
"This will help tell us how stars die. We now know that while gamma-ray bursts are very dramatic, they are really only the tip of the iceberg in terms of the energy released."
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