New research to be announced today shows strong evidence linking exploding stars called supernova to high-energy bursts of gamma rays.
The study, based on observations made in November by an international team of astrophysicists led by Shri Kulkarni of the California Institute of Technology, shows that a gamma-ray burst came from the same location as the explosion of a massive star. This is the second such finding to be released in the last six weeks.
"Our data show strong evidence that (gamma-ray bursts) come from massive stars," Kulkarni told SPACE.com.
A full account of the work will appear in an upcoming issue of the Astrophysical Journal.
Super flash
Gamma-ray bursts, also called GRBs, were first detected in the 1960s. The exotic and mysterious flashes pack the output of many galaxies into a single pulse that lasts seconds or less. If one occurred nearby, it could
Most GRBs occur well outside our galaxy, however. Because they are so brief, scientists have struggled to learn what fuels them.
In recent years, researchers have pinpointed the locations of several GRBs, leading to suspicion that they emanate from supernovae, explosions that are the natural endpoints for the lives of giant stars.
The new study examined a burst named GRB 011121, detected Nov. 11 by the Italian-Dutch BeppoSAX satellite. The event was later studied by the Hubble Space Telescope, the Australia Telescope Compact Array, the Anglo-Australian Telescope, and optical telescopes in Chile.
The scientists detected a cocoon-like remnant of the still massive central star.
What happened
Kulkarni explained what he thinks happened, a scheme that he and others have developed and refined over the past three years or so:
All stars lose matter during their lives. Stars that are more massive lose it more rapidly.
"The matter is lost via high speed 'winds' emanating from the star," Kulkarni said. "This wind results in a cocoon of matter which is well in place prior to the GRB event."
As the core of the star collapses, a dense central object forms -- possibly a black hole -- and two jets of radiation shoot out in opposite directions, along the star's axis of rotation. The jets are unsteady, and one piece of jet collides with another, generating a burst of gamma rays. Then the jet plows through the cocoon, producing a long-lived afterglow of other radiation in X-rays, visible light and radio wavelengths.
Astronomers from Poland and Chile, as well as another U.S. team from Harvard, used optical telescopes to identify the afterglow and to calculate its distance at five billion light-years from Earth.
"It is from the study of this afterglow emission (lasting months) that we were able to infer the existence of the cocoon," Kulkarni said. "The cocoon is a natural expectation of the massive-star hypothesis and thus our detection of the cocoon shows that we are on the right track."
It is not known if other GRBs are produced by the same mechanism. Some of the shortest GRBs, which last less than a second, may require a separate explanation. Collisions between dense objects such as two neutron stars or a neutron star and a black hole could be behind these other events.
"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," said Joshua Bloom, a Caltech graduate student and lead author of one of two papers on the results.
Still exotic
Another
, reported in the April 4 issue of the journal Nature, detected other signatures of a supernova around a known GRB site. Hot gas containing elements such as magnesium, silicon and sulfur was found streaming out from the location.James Reeves, of the University of Leicester in the UK, led the earlier study.
"Overall there's a large body evidence which makes the link between supernova and GRBs almost certain," Reeves said in an e-mail interview yesterday, adding that the new work by Kulkarni and his colleagues strengthens the case.
But Reeves said GRBs remain exotic, if not still a little mysterious.
"Perhaps [GRBs] are no longer so mysterious, as scientists have a good idea what can produce the large energies needed to power them, but they are certainly the most extreme physical phenomena (in terms of their total power output) known in the universe," Reeves said.
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