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New explanation offered for cosmic mystery explosions
Oct. 5, 2005
Courtesy California Institute of Technology
and World Science staff
Astronomers announced last May that they may have
uncovered the source of mysterious, huge explosions that go off every day or so in
space. But it now seems that finding may have been just part of the story.
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In this artists' conception, a neutron star
spirals inward toward a black hole. (Courtesy NASA)
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Researchers now say there may be more than one
explanation for the flashes, because the shortest of them has a different source than longer ones.
In a new set of published findings, a group of astronomers focused on these shortest explosions, and said they occur when two extremely compact stars collide.
This is the origin of the bursts that last a few thousandths of a second, according to Penn State University astronomer Derek Fox and 32 other researchers who wrote the paper published in the Oct. 6 issue of the research journal
Nature.
Longer ones, which last a few seconds, were attributed in the May 27 paper to explosions of single stars. That paper was published in the journal
Science.
The explosions, whether short or long, are called gamma-ray bursts because they briefly flash tremendous amounts of gamma rays, a type of light
much more energetic than visible light.
The findings in Nature suggest the short version of the bursts are due to violent collisions between a black hole and a neutron star,
or collisions of two neutron stars. Black holes and neutron stars are both extremely dense remnants of stars that have exploded after burning out their fuel.
The short gamma-ray bursts typically shine brighter than a billion suns during the short interval they are at peak power.
Fox and colleagues wrote that their findings were based on detailed observations of short gamma-ray bursts that occurred on May 9 and July 9 of this
year.
The May 9 event was originally detected by NASA’s Swift Satellite, and the July 9
one by High-Energy Transient Explorer, another satellite of the space agency.
Theorists had already suspected that the short bursts were caused by the extremely heavy black holes and/or neutron stars slamming together. The new observations back up that “coalescence model,” the astronomers said.
“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, one of the researchers.
The astronomers gathered evidence that they said led them to believe the July 9 event was too faint to be a supernova, or a single exploding star. It was fainter than any supernova that has ever been observed.
Further, the energies observed in both the July 9 and May 9 bursts are consistent with computer simulations for collisions of “compact object binaries”—that is, neutron stars and black holes, the researchers said.
Also, the location of the May 9 burst on the outskirts of a star-forming galaxy is at a place where one would expect to see old stars that have collapsed into the extremely massive neutron stars and even more massive black holes. The July 9 burst appears to have come from deeper within the
galaxy.
“I am amazed that we have been able to make such great strides in the space of a few months.” Kulkarni added. “Now it is time to start addressing what beast lies at the heart of these explosions.”
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