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Astrophysicists puzzle
over intergalactic coincidence

Gamma-ray burst and supernova may have no relation

artist's concept of a cosmic explosionJan. 15, 1999: Coincidences are rare in nature, so when one occurs, the natural reaction is to see if the two events are linked by more than just chance, and if that link applies elsewhere in the universe. Thus, some astrophysicists started to rethink the non-linkage between gamma-ray bursts and supernovae after an event in April 1998.

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.

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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 BATSE.COM.

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.

But if they're related, it could send astrophysicists back to rethink the mystery.

"I looked to see if there were any more of these coincidences," Kippen said. "The chances of observing a supernova within a small bit of sky over the span of a few days is pretty small, about 1 in 10,000." Kippen divided the BATSE burst catalog into two groups. First he looked for bursts that were seen both by BATSE and Ulysses. BATSE has eight detector modules. Measuring the brightness of a burst as seen by the three or four modules that are actually triggered will describe a large error box in the sky. Triangulating the arrival time of the burst with the time of arrival at Ulysses, located deep within our solar system, reduces the error box to a short, thin arc across the sky.

Left: The Compton Gamma Ray Observatory is deployed by the Space Shuttle in April 1991. Four of the eight BATSE instruments can be seen at the spacecraft's corners. The other four are on the opposite face. (NASA)

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. "

A plot of 160 recent supernova locations (red diamonds) shows that none coincides with 415 gamma-ray bursts seen by both BATSE and a detector on the Ulysses international solar polar spacecraft (purple lines). The supernova locations are shown in diamonds for clarity; their actual locations are known to much greater precision. The BATSE/Ulysses lines are the overlap between a great circle formed by timing triangulation between BATSE and Ulysses, and the rough position defined by BATSE alone. this plot also depicts the random nature of gamma-ray bursts. Because this sample covers 1991-97, SN1998bw is not shown. All of these sources are outside our galaxy. For reference, the Milky Way lies along the equator of this map. Links to 1320x615-pixel, 381K JPG. Credit: Dr. Marc Kippen, UAH and NASA/Marshall.

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.

"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."

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Author: Dave Dooling
Curator: Linda Porter
NASA Official: Gregory S. Wilson