At first GRB 000131 did not seem particularly different from other gamma-ray bursts, which are detected by spacecraft at the rate of about once a week.
"The flash of high-energy gamma radiation appeared at first to be relatively normal," explained Kevin Hurley, the University of California Berkeley physicist who operates the Interplanetary Network. "It was neither very faint, which might indicate that it had traveled a long way from its source, nor very bright, which would hint at a somewhat closer origin."
"What is important is that we have a technique to sample the deep universe in a manner not believed possible a few years ago."
The burst's unusual nature was determined only in the following days, when European astronomers, using telescopes at the European Southern Observatory (ESO) in Chile, studied the fading optical counterpart, or afterglow, to the GRB. Images of the afterglow showed it was much redder than those of other bursts, implying that the light was redshifted and thus emanated from billions of light-years away.
An accurate determination of the GRB's distance came from a spectrum of the burst taken on February 8. By that time the afterglow had faded considerably, down to magnitude 25.3, requiring a three-hour exposure with one of the four 315-inch (8-meter) telescopes of the Very Large Telescope (VLT) at ESO. In that spectrum astronomers identified a characteristic set of features known as the "Lyman-alpha forest", created when clouds of hydrogen gas absorb ultraviolet light. Astronomers found the Lyman-alpha forest not at ultraviolet wavelengths, but at much longer visible-light wavelengths, indicating that the light had been redshifted by a factor of 4.5.
This redshift corresponds to a distance of about 11 billion light-years, making GRB 000131 the most distant burst observed, beating out the previous record-holder by more than 2 billion light-years.
Because the burst is so distant, it took place in the early history of the universe -- no more than approximately 2 billion years after the Big Bang -- and thus gives astronomers another way to understand what the early universe was like.
"What is important is that we have a technique to sample the deep universe in a manner not believed possible a few years ago," said Holger Pedersen, an astronomer with Copenhagen University in Denmark. "Finding the most remote gamma-ray bursts should be a good way to trace the early phase of galaxy formation."
What the burst doesn't solve, though, is the mystery behind GRBs themselves. Some astronomers believe that such bursts are generated by hypernovae -- explosions of massive, short-lived stars that are a thousand times more powerful than a typical supernova. The difficulty in studying such bursts, particularly distant ones like GRB 000131, mean that the mechanisms behind a GRB are still poorly understood.
New spacecraft and telescopes may provide astronomers with the resources to better understand GRBs, however. NASA's High Energy Transient Explorer 2 (HETE 2) spacecraft, launched earlier this month, will soon alert astronomers to GRBs within 10 to 20 seconds of their detection, allowing telescopes such as the VLT to observe the optical afterglow when it is far brighter. "If observations can start within, say, five minutes, optical light may then be many thousand times brighter than we saw it in GRB 000131, allowing for much more detailed observations," said Pedersen.
Such techniques may also aid astronomers in discovering even more distant bursts. "The observation of this burst can be viewed as another rung in the distance ladder," said Hurley. "Everything about gamma-ray bursts has proven to be a complete surprise to us, and this observation is a good example. Our data may indicate that we can indeed see much farther still."
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