Astronomers Find Rare Beast By New Means
For the first time, astronomers have found a supernova explosion with properties similiar to a gamma-ray burst, but without seeing any gamma rays from it.
The discovery, using the National Science Foundation's Very Large Array (VLA) radio telescope, promises, the scientists say, to point the way toward locating many more examples of these mysterious explosions.
"We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites," said Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics.
The telltale clue came when the radio observations showed material expelled from the supernova explosion, dubbed SN2009bb, at speeds approaching that of light. This characterized the supernova, first seen last March, as the type thought to produce one kind of gamma-ray burst.
"It is remarkable that very low-energy radiation, radio waves, can signal a very high-energy event," said Roger Chevalier of the University of Virginia.
When the nuclear fusion reactions at the cores of very massive stars no longer can provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star's material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a "core-collapse supernova" as the cause of one kind of gamma-ray burst.
Not all supernovae of this type, however, produce gamma-ray bursts. "Only about one out of a hundred do this," according to Soderberg.
In the more-common type of such a supernova, the explosion blasts the star's material outward in a roughly-spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.
The superfast speeds in these rare blasts, astronomers say, are caused by an "engine" in the center of the supernova explosion that resembles a scaled-down version of a quasar.
""Engine-driven" supernova explosion with accretion disk and high-velocity jets. (Credit: Bill Saxton, NRAO/AUI/NSF)"
Source: National Radio Astronomy Observatory
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