On April 25, 1998, a different kind of supernova ended with a bang instead of a whimper. Above left, a 1985 image of galaxy ESO 184-G82 by the United Kingdom Schmidt Telescope in Australia shows nothing unusual. Above right, a May 4, 1998, image by ESO's 3.58-m New Technology Telescope (NTT) shows SN1998bw, the very bright, bluish star at the center, (arrow), located in an arm of the galaxy. The nova gave observers a classic situation -- evidence that points in different directions and dares anyone to find a single answer. "All of this may lead to a revolution in our thinking about how core-collapse supernovae are produced," wrote Dr. Eddie Baron of the University of Oklahoma in the Oct. 15 issue of the prestigious science journal, Nature. Questions to be answered include what causes "ordinary" supernovae, is there a limit in their energy release, and when does core collapse cause a gamma-ray burst? The full story is covered in Science@NASA's story, When stars go hyper. Images courtesy European Southern Observatory. Links to 463x365-pixel, 44KB JPG. |
So far, says Dr. Marc Kippen, the odds are against gamma-ray bursts being associated with supernovas.
"They might be related," he said, "but then you must explain how a local supernova produced a gamma ray burst that looks like all the other ones that evidently come from very great distances."
Kippen, an astrophysicist with the University of Alabama in Huntsville working at NASA's Marshall Space Flight Center, published the results of his analysis in the Oct. 10 1998 issue of The Astrophysical Journal.
"We can almost conclusively say that no bright gamma-ray burst detected so far comes from a known supernova," Kippen said. "We are less certain about weaker bursts because they can't be precisely located. In addition, we miss most supernovas, so about 10 percent of the weaker gamma-ray bursts could come from supernovas."
Gamma-ray bursts have been one of the most mysterious phenomena in the universe since their discovery about 30 years ago. Initially they were thought to be associated with neutron stars within our galaxy. Observations with the Burst and Transient Source Experiment (BATSE) board the Compton Gamma Ray Observatory, launched in 1991, have shifted the scene from our galaxy to deep in the universe.
BATSE scientists found that gamma ray bursts are randomly distributed across the sky, indicating that they are peppered throughout the heavens, rather than clustered along the plane of our Milky Way galaxy, which would be expected if they were part of the galaxy.
Since 1997, a few bursts have been observed with optical, X-ray, or radio counterparts that are thought to be at cosmological distances up to 10 to 12 billion light years away. Thus, the original explosion had to be incredibly powerful to produce a flash of gamma radiation that is readily detected at Earth.
Then came a burst and a supernova both on April 25, 1998, both in the same region of the sky. The gamma-ray burst was observed by BATSE and the Beppo SAX satellite. The precise position of the burst provided by instruments on Beppo SAX allowed ground-based optical telescopes to discover that the burst was coincident with a new supernova -- SN1998bw -- within the same small section of the sky.
This presented a new challenge because the gamma-ray burst was average in its properties. Nothing distinguished it from the other bursts routinely detected by BATSE. However, sn1998bw was extraordinary -- the intrinsically brightest supernova ever observed in its category.
It could be a coincidence. BATSE has recorded more than 2,000 bursts since 1991, about one a day. And only 1 supernova in 10 is actually detected. So, a burst and a supernova are bound to coincide in time and apparent location.
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Kippen then compared these with supernovas that had been detected at about the same time as the burst. He allowed a generous margin -- up to a month -- since the dates of supernova explosions aren't always precisely known.
He came up empty handed. The 415 bursts and 585 supernovas all had separate locations.
Still, there is the possibility that the supernovas might cause weaker bursts that were detected by BATSE and not by Ulysses which carries a much smaller instrument. This gave him a set of 1,222 bursts.
"At some level you expect gamma rays to come from a supernova," Kippen said. But they should be less powerful than the events that cause gamma-ray bursts at cosmological distances, and their light profiles -- how they brighten and dim -- also should be different.
"The result for using just BATSE locations was that there's no significant excess," Kippen said. "It was consistent with random locations. "
The issue likely will remain open for some time, until the right combination of instruments happens to be pointing in the direction of a supernova that also yields a burst, or until more advanced telescopes are built.
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"The next-generation gamma-ray detectors could have the capability of observing hundreds of bursts per year with very accurate positions," Kippen said. "Then you could start to make more definitive statements. Then we could also precisely localize weak bursts and say whether any of them occur with supernovas."
In the meantime, astrophysicists are left with three possibilities.
"The first is that this is just a random coincidence," Kippen said.
The second is that the burst and supernova are related. "Then you have to explain why a supernova produced burst that looked just like all the other ones seen at presumably very large distances," he continued.
The third possibility is that most bursts occur at great cosmological distances, but a small percentage -- probably less than 6 percent -- are produced by a different mechanism in our galactic neighborhood.
For now, scientists may have to live with an uncomfortable coincidence.
"One event is just one event," Kippen said. "It's not overwhelming evidence."
Web Links
The enigmatic fingerprints of gamma-ray bursts Jan. 8, 1999. "Hard" and "soft" colors put gamma-ray bursts in a different light
Gamma-ray Bursters cross the 'Line of Death' - Oct. 13, 1998. A study of gamma ray burst spectra shows one more thing that these mysterious, cosmological gamma ray bursts are not.
Blast from the past: the latest clue in solving the gamma-ray burst mystery (May 6, 1998).
Gamma-ray burst identification earns top prize (Jan. 12, 1998).
Discovery may be "smoking gun" in gamma-ray mystery (March 31, 1997).
Twinkle, twinkle, massive fireball -- reports from the 4th Huntsville Gamma-ray Burst Symposium (Sept.17,1997)
Learn more about cosmic gamma-ray bursts at http://www.BATSE.COM.