NSF PR 97-37 - May 15, 1997
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Very Large Array Detects Radio Emission from Gamma-Ray
Burst
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.
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