More News from the Gamma Ray Burst Symposium
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September 17, 1997
Return to the Hubble story.
High Energy and Higher Energies
Gamma ray bursts are seen about once a day at totally random locations in the sky, and scientists have had great difficulty in finding their sources. One method they use is to dissect the data on each burst and try to figure out what patterns they have in common - or in what ways they might differ.
One key pattern was described by Dr. Geoff Pendelton of the University of Alabama in Huntsville. Pendleton, working with a team comprising scientists at UAH, Marshall, and Mankato State University in Minnesota, found that bursts tend to possess two different types of emission, dubbed "high energy" and "non-high energy," reflecting the nature of the emission characteristics.
BATSE records data in four distinct energy channels. What Pendleton classifies as high-energy emission is seen in all four of these channels. Non-high-energy emission is seen only in the first three; with virtually no energy detected above 300,000 electron volts (300 keV, about 300,000 times more energetic than visible light). What is exceptional about Dr. Pendleton's result is that both types of distinct emission can be seen in one burst, indicating that it is unlikely that different astrophysical objects are responsible for the two "flavors" of burst emission. "It looks to me like the same bursting source can make both high-energy and no-high-energy emission," remarked Dr. Pendleton.
Scientists can browse through a catalog of 1,637 bursts which are recorded on a CD-ROM distributed to the conference attendees.
"There's a lot of science hidden in there," said Dr. Charles Meegan, a conference organizer and a BATSE co-investigator. "You're all invited to dig it out." The catalog covers every burst recorded by BATSE from its launch in 1991 through August 1996, the cutoff date for the current collection known as the 4B catalog. 4B contains 515 more bursts than 3B, and the total is growing. Meegan said that BATSE has seen more than 1,900 bursts to date, and will see their millennial burst - number 2,000 - by the end of the year.
Clicking the diagram at right will take you to a simulation of the interaction of a gamma-ray photon with the BATSE instrument - the interaction which triggers the recording of a gamma-ray burst which is subsequently recorded in the catalog.(0.44MB)
Even more bursts than that lie buried in the data. Jeff Kommers of the Massachusetts Institute of Technology described how he has mined the BATSE data to find another 438 burst events between August 1992 and August 1995. The bursts are not in the catalog because they are below the "trigger threshold," a predetermined brightness level in two or more detectors. Some fainter events don't quite meet this criterion and are "missed" by the experiment. BATSE is continually bombarded with radiation that registers in each of the eight detectors all the time, so the on-board electronics were designed to sound the alarm only when a burst triggered at least two detectors simultaneously with a brightness above a predetermined threshold.
Even though there may not be triggering events, all the data are recorded, since additional findings might be hidden below the instrument's triggering threshold. That's where Kommers hunted for "non-trigger" gamma ray bursts, events that would qualify as a burst but which were not quite strong enough to trigger the experiment. Kommers had to weed out solar flares, cosmic rays hitting Earth's atmosphere and releasing gamma rays, effects from Earth's own magnetosphere, and other effects.
"Our search has fundamentally expanded BATSE catalog," Kommers said. "There are 30 to 40 percent more bursts, and they extend downward to lower peak fluxes," meaning fainter brightnesses than what BATSE routinely detects. Equally important, these new bursts appear to be as randomly scattered cross the sky as those "triggered" bursts in the 4B catalog.
For additional information during the symposium,
call Kelly McFalls at the symposium newsroom at 205-518-9424 or 518-2446.
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