<!--intro-->Astronomers still don't know where gamma ray bursts come from, but NASA researchers have concluded that they may originate from at least two different types of cosmic explosions.<!--/intro--> Dr. Jay Norris of NASA Goddard Space Flight Center in Greenbelt, Md., analyzed several hundred gamma-ray bursts, mysterious flashes of light more powerful than any known explosion other than the Big Bang. He found that the shorter bursts, lasting less than two seconds, have different characteristics than longer bursts, and that the difference is like night and day: Bursts just under two seconds and just over two seconds appear completely different. The characteristics of the short burst and the abrupt change after two seconds suggest that the two types of bursts may come from very different types of sources. Norris presents these findings today at a press conference at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Honolulu, Hawaii. "The source of gamma-ray bursts remains one of the great mysteries in modern astronomy," said Norris. "All gamma-ray bursters need not be the same type of object. The evidence is growing that shorter bursts are very different." Gamma-ray bursts can appear from any direction without warning and may last only a few milliseconds to just over a thousand seconds. Orbiting satellites have indicated several occur per day, on average. In their brief moment of glory, the bursts outshine the entire Universe in gamma rays before fading to oblivion. Scientists conjecture that the bursts come from merging compact objects such as black holes or neutron stars or perhaps from a massive star explosion, called a hypernova. A single gamma ray burst might comprise thousands of gamma-ray photons (particles of light, in this case, high-energy gamma rays). Norris and his colleagues -- Drs. Jeffrey Scargle (NASA Ames Research Center) and Jerry Bonnell (NASA Goddard) -- examined gamma-ray burst time histories. These time histories are composed of gamma-ray pulses, a group of photons arriving at about the same time. The team counted the number of pulses in each burst. They also measured arrival time (to the gamma-ray detector) of lower-energy and high-energy pulses. Lower-energy pulses lag behind high-energy pulses. Norris said the analysis demonstrates that short bursts have significantly fewer pulses and that their lag times are 20 times shorter than the lags in the longer bursts. Dramatically fewer pulses and shorter lags suggest that the short bursts are produced in physically different objects. Further, the lag distribution in short bursts does not appear to be related to other factors such as the peak brightness of the burst. In contrast, Norris et al. have demonstrated in the long variety of gamma-ray bursts that peak luminosity is related to lags. The team's findings dovetail with the work of Dr. William Paciesas of the University of Alabama, Huntsville, who recently found further evidence that short bursts are spectrally different from long bursts. Because short bursts last for at most a couple of seconds, they are more difficult to study than longer bursts. In fact, unlike longer bursts, the shorter bursts have no detected afterglow in X-rays or visible light. Observations of long burst afterglows suggest those bursts are associated with star forming regions in distant galaxies. "We really know very little about short gamma-ray bursts," said team member Jerry Bonnell. "We haven't yet identified an afterglow from a short burst and so have only the gamma-ray data to study." Bonnell speculated that these shorter bursts might be binary neutron star mergers, a model that has fallen into disfavor for the source of long gamma-ray bursts. If there is an afterglow from shorter gamma-ray bursts, two NASA satellites might help detect them. The High Energy Transient Explorer (HETE-2), launched on October 7, monitors the skies for gamma-ray bursts and quickly relays the burst location to orbiting and ground-based telescopes for follow-up study of the afterglow, which for longer bursts may linger for weeks or months. The Swift satellite, scheduled for launch in 2003, will add even greater precision to the study of gamma-ray bursts.