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Right: The "color-color" diagram at right links to a 202K animated GIF that depicts the evolution of the burst's hard:soft ratio. What's in a name? Bursts are designated by when they occur (two digits for year, month, date) instead of the location designator normally used for stars. So, this burst was observed May 3, 1991. (Tim Giblin, UAH & NASA) Gamma-ray bursts mystified scientists since they were discovered in the late 1960s by military satellites monitoring a nuclear test ban treaty. At first they debated whether the bursts were from within or near our galaxy - or far away. Observations by the Burst and Transient Source Experiment (BATSE), launched in April 1991, helped establish that most bursts come from cosmological distances - billions of light years from Earth. That discovery directed scientists to another mystery: What could cause so bright a blast at so great a distance? The answer is yet to be found, but scientists at NASA's Marshall Space Flight Center may have developed a way to lift spectral "fingerprints" that may help find the culprit.
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) Giblin will outline his study today at the American Astronautical Society's annual conference in Austin, Texas. "I'm trying to identify classes of bursts - if they exist - by examining their spectral properties over time," Giblin said, "and to look for spectral behavior that we have not seen before." Scientists have found no clear classification scheme for gamma-ray bursts. If one can be found, it would help in directing studies to determine an even bigger mystery, What causes bursts? In his quest, Giblin is applying the four energy channels or "colors" that BATSE can see in the "Color-Color Diagram" method used in other branches of astrophysics. |
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BATSE does more than simply detect a burst. Among its many capabilities, BATSE measures a burst's intensity every 64 milliseconds (about 156 times a second) in four energy channels: 25-50 keV, 50-100 keV, 100-300 keV, and 300-2,000 keV (or, 2 MeV). Visible light, by comparison, is centered around 3 eV. Often, a burst fades fastest in the highest energy output than in its lower energy output. Giblin and his colleagues at NASA/Marshall had suspected that there was more to it than simply flaring and fading. |
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Giblin compared a burst's brightness at different energies - the ratios of channel 2 to 1, and of channel 4 to 3 - as the burst evolved with time. It's like striking a match and comparing its green-vs.-red brightness (the "soft" color ratio) against its ultraviolet-vs.-blue brightness (the "hard" color ratio). In a sense, Giblin is measuring how "red" or "blue" the bursts are at different times in their histories. This "color-color" diagram is plotted in colors that indicate the progression of the burst through time, from blue at the start of the flash to green at midpoint to red at the end. This makes it easier for the eye to track the burst's progress in time. What Giblin found was that the 50 brightest gamma ray bursts formed several distinctive patterns. The most common, occurring in 36 percent, is a crescent. The second largest group (18 percent) formed islands and flats ("blobs," Giblin said.). Other groups are a loop (10 percent), an ear with a lobe (7 percent), and a bar (9 percent). About 21 percent formed no discernible pattern. |
 Five distinct patterns have emerged for color-color diagrams of gamma-ray bursts. Links to 982x537-pixel, 178K JPG. (Tim Giblin, UAH & NASA) In some cases, the crescent pattern actually is a double crescent, Giblin explained. As the burst declines, it more or less doubles back on the trace it left as it rose. Others, such as the lobe, indicate that the burst's color is "harder" - it is stronger at the higher energies - at its start than during its finish. Looking at the bursts in greater detail reveals that some crescents trace a sawtooth pattern inherent to the highly variable nature of the burst. Just what the different patterns mean in terms of the burst source remains to be seen. Giblin is running computer simulations that include evolution of the spectral model and to look at the time profiles in an artificial pulse. Giblin found that different pulse shapes produce different patterns in the color-color diagram. "Gamma ray bursts are really made of a series of pulses, each having its own spectral behavior," Giblin said. "I think this trace is telling us that these events are more complex than we thought." |
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"The idea is to look for
patterns in how the bursts brighten and dim in different parts
of the energy spectrum," said Tim Giblin, a University of
Alabama in Huntsville graduate student working at NASA/Marshall.
"It is important in this field to find subclasses of bursts."