Astronomers have used the National Science Foundation's (NSF) Very Large Array (VLA) radio telescope to make the first detection of radio emission from a cosmic gamma-ray burst. This sheds the first light on long-standing questions about the actual physics of these mysterious, tremendously energetic events.
"The mere discovery of radio emission from this gamma-ray burst rules out some theoretical models," said Dale Frail of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico. "We are still observing it and each additional observation will help further discriminate among competing models."
"This detection may finally tell us what these mysterious gamma-ray bursts are, helping to resolve one of the biggest mysteries in astrophysics," said Hugh Van Horn, director of the NSF's division of astronomical sciences.
The VLA detection was made by some of the same scientists who announced yesterday that optical observations showed that gamma-ray bursts come from great distances. In addition to Frail, the VLA astronomers are: Shri Kulkarni of Caltech and the BeppoSAX Gamma-Ray Burst Team, consisting of Luciano Nicastro, Eliana Palazi, Enrico Costa, Marco Feroci, Luigi Piro, Fillipo Frontera, and John Heise.
The burst of gamma rays was detected May 8 by the Italian-Dutch satellite BeppoSAX. Hundreds of such bursts have been recorded by satellites in the past 30 years, but last week's event already has become the most scientifically significant of them all. For years, the difficulty of precisely locating the bursts' position in the sky made it nearly impossible to study them with optical and radio instruments. In late 1996, this situation improved with the launch of BeppoSAX, which can pinpoint the bursts' location much more accurately than previous spacecraft. Following BeppoSAX discoveries, optical and radio astronomers have been able to make quick observations of the burst locations.
The largest unanswered question about gamma-ray bursts has been their distance from Earth. Scientists debated whether they came from unseen objects within our own Milky Way Galaxy or from other galaxies billions of light-years away. A team of astronomers from Caltech and NRAO, using the 10-meter W.M. Keck Telescope in Hawaii, found visible light coming from the May 8 gamma-ray burst. They announced yesterday (May 14) that analysis of its optical spectrum showed its minimum distance to be about seven billion light-years away, thus resolving the distance question.
The VLA was trained on the May 8 burst less than four hours after the burst's discovery, but no radio emission was found until May 13. A repeat VLA observation early on May 15 showed that the radio emission is increasing in intensity. The May 15 VLA observations also show that the radio emission increases in intensity at shorter radio wavelengths. Frail and his colleagues will continue to observe the object with the VLA, and also plan to quickly use the continent-wide Very Long Baseline Array (VLBA) to make extremely high-resolution images.
"By tracking changes in the radio intensity at different wavelengths and getting very high resolution images, we hope to gain unprecedented knowledge about a class of objects about which -- just last week -- we knew almost nothing," Frail said. "The Keck observations showed that they are very distant. We still have no idea about what kind of parent objects give rise to gamma-ray bursts. With our planned VLBA observations, we hope to get at the heart of this problem."
The VLA and VLBA are instruments of the National Radio Astronomy Observatory, a National Science Foundation facility operated under cooperative agreement by Associated Universities, Inc.