Listen to the UVA Today Radio Show report on this storyby Fariss Samarrai:
January 28, 2010 — For the first time, astronomers have found a supernova explosion with properties similar to a gamma-ray burst, but without detecting any gamma rays from it. The scientists say the discovery, using the National Science Foundation's Very Large Array radio telescope, located near Socorro, N.M., promises to point the way toward locating many more examples of these mysterious explosions.
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 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," noted University of Virginia astronomer Roger Chevalier, a co-author of the study, which appears today in the journal Nature.
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.
Understanding these explosions is important to better understanding the evolution of the universe, Chevalier said, because they provide the energy that causes stars to form and the heavy elements that make up the planets and, in fact, everything.
For the past decade or so, astronomers have identified one particular type of "core-collapse supernova" as the cause of one kind of gamma-ray burst. Only about one out of 100 supernovae produce gamma-ray bursts.
In the more common type of 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, a small amount of the ejected material is accelerated in jets to nearly the speed of light. The jets burrow through the star and escape out of the surface.
The superfast speeds in these rare blasts, Chevalier and his colleagues say, are caused by an "engine" in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.
Until now, no such "engine-driven" supernova had been found any way other than by detecting gamma rays emitted by it.
This discovery by radio emission observation, rather than through gamma rays, is a breakthrough, Chevalier said. And more such discoveries soon will be possible, he added, when the Very Large Array is refurbished in the next few months.
"The community of radio observers likely will be able to make these observations more frequently, in deeper detail, as the Expanded Very Large Array comes on line," he said.
Why didn't anyone see gamma rays from this explosion?
"We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen," said lead author Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics. In that case, finding such blasts through radio observations will allow scientists to discover a much larger percentage of them in the future.
"Another possibility," Soderberg added, "is that the gamma rays were 'smothered' as they tried to escape the star. This is perhaps the more exciting possibility, since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites."
One important question the scientists hope to answer is just what rare property causes the difference between the "ordinary" and the "engine-driven" core-collapse supernovae. This is important, Chevalier said, because it is a crucial clue to the life and death of massive stars, which play an essential role in the evolution of the universe.
Chevalier and Soderberg worked with Alak Ray and Sayan Chakrabarti of the Tata Institute of Fundamental Research in India; Poonam Chandra of the Royal Military College of Canada; and a large group of collaborators at the Harvard-Smithsonian Center for Astrophysics.