Oct. 18, 2000 The afterglow of a gamma-ray burst in the southern constellation of Carina - more distant than any high-energy flare ever observed - has been detected by a network of spacecraft spread over the solar system, and has been traced back to its original explosion about 11 billion years ago.
The observations, made by a cluster of interplanetary probes called the Interplanetary Network, revealed that the burst probably came from a gigantic dying star more than 30 times as massive as the sun, when the universe was about one-tenth of its present age, said Kevin Hurley, a physicist at the Space Sciences Laboratory of the University of California, Berkeley, and principal investigator of the gamma-ray burst experiment on board NASA's Ulysses spacecraft. Ulysses is one of several spacecraft studying the sun, and the only one that passes over the sun's poles.
"Detection of gamma-ray burst GRB 000131 at an extremely high red shift of 4.5 corresponds to a distance of about 11 billion light years away," said Hurley, whose results will be reported today at the Science of Gamma-Ray Bursts conference in Rome, an international colloquium on gamma ray bursts. "The light from this gigantic flash had traveled 11 billion years before reaching the Earth, and suggests that these explosive objects may provide us with the longest yardsticks yet for detecting and studying galaxies in the early universe."
Hurley's colleagues also will report details of the gamma ray observations at a press briefing today in Rome. Results of the observations will appear in a December issue of the European science journal Astronomy and Astrophysics.
All objects in the universe are flying away from each other as a result of the Big Bang. More distant galaxies recede faster from Earth than nearby galaxies. This is observed as a Doppler effect, best explained by the pitch of a train whistle, which rises as the train approaches and diminishes as the train speeds away. Similarly, if a galaxy is approaching the Earth, all of the wavelengths in the galaxy's spectrum are shifted toward the blue end of the spectrum. If the galaxy is receding from Earth, all wavelengths in the spectrum are shifted toward the red end in what is termed a "red shift." Observations of GRB 000131 revealed that it is receding and its light is shifted toward the red end of the spectrum by a factor of 4.5, which is a significant shift, Hurley said.
Before these observations were recorded, the most distant gamma-ray burst to be detected was GRB 971214, estimated to be less than 9 billion light years away in the constellation Ursa Major. A violent burst of gamma-ray radiation from this object was recorded on December 14, 1997, by Italian and U.S. satellites, including Ulysses.
"If this were the Olympics, we'd have the gold medal now," said Hurley, who runs the network of spacecraft, which includes the joint European Space Agency/NASA Ulysses spacecraft, NASA's Near Earth Asteroid Rendezvous (NEAR-Shoemaker) and Wind spacecraft, and the Italian Bepposax spacecraft. "Still, the fact that such a faint, distant source can produce a burst of gamma rays which appears to be of more or less average intensity from Earth hasn't been explained yet."
Gamma-ray bursts are mysterious flashes of high-energy light occurring typically about once a day somewhere in the sky. However, their origins remain a mystery to astronomers, most of whom believe they are enormous explosions that occur far across the universe. Only recently has enough evidence been accumulated to link the longest of these bursts to hypernovas: giant, extraordinarily intense, unusual supernovas, which are dying stars that collapse under the weight of their own mass.
At their peak, these bursts are by far the brightest emissions of gamma-ray radiation in the sky, Hurley said. In fact, gamma-ray bursts are the most powerful explosions in the known universe. They generate more energy in a few seconds than the sun will generate in its entire lifetime.
Bursts are detected approximately once a week by these surveillance craft, although the Italian Bepposax spacecraft did not detect this burst. All four spacecraft are equipped with detectors to measure gamma rays, an energetic form of x-radiation, Hurley said. The gamma rays are impossible to detect from Earth because they are absorbed by the atmosphere. By timing the arrival of gamma rays as they reach the spacecraft, their arrival directions can be precisely determined.
"Although this distant burst was observed on January 31, 2000, it has taken scientists eight months to study its source," Hurley said. "The flash of high-energy gamma radiation appeared at first to be relatively normal. 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."
However, when a team of Finnish and Danish astronomers observed the object with the European Southern Observatory's Very Large Telescope in Chile, they found a very dim optical afterglow, suggesting that it was indeed situated at an extremely large distance. To confirm this suspicion, they had to observe it repeatedly, adding the results of each observation to previous observations until the picture became clear.
Hurley and his colleagues believe a sharply focused beam of gamma-ray radiation probably made the observations possible. "It seems likely that the gamma rays were emitted in a narrow beam, like a searchlight, and we just happened to be in that line of sight," Hurley said. "The faintness of the optical radiation, which was not beamed, was indicative of its large distance."
Since gamma radiation is energetic enough to travel unimpeded throughout the entire universe, astronomers are excited about the prospects of using gamma-ray bursts and related optical observations to reveal the details of very distant galaxies and matter in the universe.
"Optical light traveling through extragalactic space on its way toward Earth changes in various ways," Hurley said. "Atoms of hydrogen can absorb part of it along the way and galaxies can actually bend the light. The effect that matter and galaxies in the very distant universe have on this distant light is one of the factors that makes the study of gamma-ray bursts so interesting."
Some theories suggest that gamma-ray bursts might be detectable out to red shifts of 10 or more - about two times farther out than the Jan. 31 gamma-ray burst - and, thus, act as a window on the very early universe.
"The observation of this burst can be viewed as another rung in the distance ladder," Hurley said. "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."
The international Science of Gamma-Ray Bursts conference is being hosted by Italy's Consiglio Nazionale di Ricerche, the Italian national research organization that sponsors almost all scientific research in Italy.
Sponsors include the Istituto di Astrofisica Spaziale (Space Astrophysics Institute) in Rome; the Istituto di Technolgie e Studie delle Radiazioni Extraterrestri (Institute for Technology and the Study of Extraterrestrial Radiation) in Bologna; and the European Southern Observatory, a consortium of eight European countries that operates the world's most advanced multi-telescope observatory in Chile.
Images and the latest results from the Ulysses mission and other scientific probes studying the sun are available at: http://ssl.berkeley.edu/ipn3/index.html; "http://ulysses.jpl.nasa.gov/; and at http://sci.esa.int/home/ulysses.index.cfm.
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