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Oct 21, 1999: Three decades after the discovery of gamma-ray bursts, their causes remain elusive and still offer potential pitfalls for investigators. "We really need to be careful ... and make sure that we're interpreting the meager data carefully," said Dr. Gerald Fishman of NASA's Marshall Space Flight Center. Speaking at the Fifth biennial Huntsville Gamma Ray Burst Symposium, Fishman reviewed a number of the questions that astrophysicists still have to resolve in what has moved from an oddity to a hot topic. Gamma-ray bursts were discovered by accident in the late 1960s when the United States launched satellites intended to monitor compliance with the nuclear test ban treaty. They discovered bursts of gamma radiation that came from outside our solar system. Initially these were thought to be associated with our galaxy. But they came and went so quickly that locating bursts was impossible. Right: The Vela 5b satellite, launched in 1965, was one of a series of satellites that recorded the some of the earliest extra-galactic gamma-ray bursts ever seen. With the launch in April 1991 of the Compton Gamma Ray Observatory, scientists hoped to identify the burst sources. Instead, Fishman's Burst and Transient Source Experiment (BATSE), one of the observatory's four instruments, soon showed that the bursts were evenly scattered across the heavens. Combined with the fact that there were fewer weak bursts than anticipated, the discovery implied bursts originated from near the edge of the observable universe.
(A gentle reminder of that earned a laugh when Fishman put up a plot of the 2512 bursts BATSE has recorded. "For historical reasons we plot this in galactic coordinates," he said, since even he expected them to have galactic origins. "Perhaps we should change it to celestial coordinates.") Yet even with more than 2,500 bursts recorded by BATSE, and more than a dozen apparent burst sources captured by optical and other telescopes, mystery still shrouds the cause and caution must be used in interpreting the data. For example, Fishman said, astrophysicists sometimes refer to statistically significant spectral lines appearing in the data. Above: Because scientists originally believed gamma-ray bursts came from within our galaxy, location maps for GRBs have always been plotted in galactic coordinates. However, scientists have since discovered GRBs are distributed isotropically, that is, uniformly throughout the sky, indicating a cosmological origin. "We have yet to see a gamma-ray feature that shows up consistently among detectors," Fishman said. "That says to us that we don't understand the systematic errors in our own system." BATSE actually comprises eight detector modules, each pointing out from the face of an imaginary octahedron or double pyramid. At least two must detect an event before the system is "triggered" to announce a burst. Most events are seen by at least three and sometimes four of the detectors. While the modules are nearly identical, individual variations cause slight differences in how the bursts are measured. Left: One of the 8 BATSE detector modules. This is a "structural test model," built to determine whether the design is rigorous enough for space flight. A candidate for the cause of gamma-ray bursts is exceptionally violent supernovae or hypernovae. They would have similar energies, and both could involve energy beamed like water from a fire hose. "We see tantalizing results, but none are definitive," Fishman cautioned. Another reported finding is that short-duration gamma-ray bursts are anisotropic, meaning they are more numerous in one area. (A good example of anisotropy is our night sky: most stars are seen in the Milky Way, our view across our galaxy.) Not so, Fishman said. Short bursts are isotropic - evenly scattered across the sky. Meanwhile, scientists have their hands full interpreting the wide variety of burst shapes, the intensities that rise and fall with time. Some are very strong and brief and have no fine-time variations, like a photographer's flash going off, while others have a spiky structure. There are FREDs - fast rise and exponential decay - that appear like a struck match flaring and then fading away. There are odd bursts like one that have a precursor followed 100 seconds later by the main event. Right: A sample light curve showing a typical profile for a gamma-ray burst. Links to 640x480 pixel image. "It's trying to tell us something," Fishman said. "We're just not smart enough to understand it yet." Some of the uncertainty comes from how BATSE modules detect and measure bursts, admitted Dr. Charles Meegan, also of NASA/Marshall. These "self-inflicted biases" can make the work more challenging. For example, "slow risers" can be missed if the burst rises slowly enough that the BATSE electronics think it's a variation in the background noise. Or the apparent peak flux can vary. Whether a burst produces 100 counts in 100 milliseconds or 1 second, the electronics will trigger since they are keyed to 100 counts in 1,024 milliseconds (1.024 sec). Meegan said that the BATSE team is working to measure the biases. Nevertheless, BATSE and other instruments such as the Dutch-Italian Beppo-SAX and Japanese ASCA satellites have produced a wealth of information allowing scientists to try peeling back some of the layers of mystery surrounding gamma-ray bursts. These include:
"Power laws are eagerly sought in astrophysics because they tend to say something is happening in a particular way," Preece explained. Most bursts observed by BATSE plot as two power-law lines, one rising, and one falling. The great majority intersects around 220 keV. A speaker will offer possible meanings for that number later in the week. |
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