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Radio Astronomers Reveal "Bizarre" Behavior of Gamma-Ray Burster

Radio astronomers today revealed that the first gamma-ray burster ever detected at radio wavelengths has surprised them by its erratic behavior. "We expected the burster to act at radio wavelengths much as it does at X-ray and visible wavelengths -- that is, rise in brightness, then slowly become weaker," said Dr. Dale Frail of the National Radio Astronomy Observatory (NRAO). "Instead, it has completely surprised us." The announcement was made at the American Astronomical Society meeting in Winston-Salem, NC.

Frail and Dr. Greg Taylor, also of NRAO, used the National Science Foundation's Very Large Array (VLA) and Very Long Baseline Array (VLBA) radio telescopes to study the gamma ray burst which exploded on May 8. The VLA was turned on the region of the burst within four hours of its discovery by the new orbiting Italian-Dutch observatory, Beppo-SAX. The radio astronomers have observed the object on a near-daily basis since then, and that monitoring has revealed the burster's unexpected behavior.

"It is remaining at a roughly steady level of brightness, but has occasional flares in which it brightens by factors of two to three," Frail said. These variations are seen not only by the VLA, but also by the VLBA, a continent-wide radio telescope system that shows the object to be less than a few light years in size. The VLBA observations show a level of detail more than 50 times greater than that of the Hubble Space Telescope at optical wavelengths.

For years, positions of gamma-ray bursts were known only to within several degrees in the sky, because of the limitations of earlier gamma-ray telescopes. Using the VLBA, the astronomers now have pinpointed a position for the May 8 burst to within a thousandth of a second of arc. "In only a year, this field of research has progressed to the point that we have a position more than a million times more accurate than before," Taylor said. This has allowed the researchers to show that the burster has not moved in the month since its discovery.

What causes the unexpected flares in the radio output of this object? "One idea is that we are seeing the radio analogy of stars twinkling in the night sky," Frail said. This would require the object to be very small. Another idea is that the variations in the radio emission are not connected to the burst itself, but are arising within an active galaxy in which the burst occurred.

"In order to solve the 30-year mystery of what causes gamma-ray bursts, we need to know what kind of galaxies give rise to them," Frail said. Both the VLA and VLBA will continue to observe this burster to gain additional information about its behavior in the coming months. "Theorists have made detailed predictions about how the radio intensity of such objects should behave over time. The ongoing VLA monitoring will test those predictions. Similarly, the great resolving power of the VLBA may allow us to track changes in the structure of the object over time."

Gamma ray bursts were first discovered in the 1960s, and have mystefied scientists since then. Because the positions of these bursts could not be well determined before the launch of BeppoSAX, it was difficult for optical and radio astronomers to follow up on the gamma-ray detections. A longstanding controversy arose over whether the explosions creating these powerful bursts of radiation are occuring within our own Milky Way Galaxy or in other galaxies perhaps billions of light-years distant. Optical studies of the May 8 burst indicate that it is at least 7 billion light-years away, thus apparently resolving the distance question. The exact nature ot the explosions is still a question for further study.

The VLA and VLBA are instruments of the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.