Embargoed until 2 P.M., EDT
NSF PR 97-53 - September 17, 1997
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Radio Observations Provides New Clues to Nature of
Gamma Ray Fireball
A team of astronomers using a pair of National Science
Foundation (NSF) radio telescopes has made the first
measurements of the size and expansion of a mysterious,
intense "fireball" resulting from a cosmic gamma ray
burst last May.
After three months of observations with the NSF's
Very Large Array (VLA) and Very Long Baseline Array
(VLBA) radio telescopes, scientists have learned that
the "fireball" of debris expands very closely to the
speed of light. They estimate its current size to
be about one-tenth of a light-year, or 170 times the
distance between the sun and Pluto. The scientists
reported their findings about the May 8 gamma ray
burst in the September 18 issue of Nature.
"For 30 years, we've known almost nothing about these
mysterious explosions in the sky. Our observations
show that these events release truly incredible amounts
of energy," said Dale Frail, of the National Radio
Astronomy Observatory (NRAO) in Socorro, New Mexico.
"It was only a few months ago that our observations
showed that such bursting objects are located far
beyond our own galaxy. However, astronomers had little
evidence for how this cosmological juggernaut actually
works. The radio observations have revealed a size
of the fireball, unobtainable by any other technique,
thereby enabling astronomers to learn about inner
workings of such objects," said Shri Kulkarni, professor
of astronomy at the California Institute of Technology
(Caltech).
In addition to Frail and Kulkarni, the astronomers
are Greg Taylor of NRAO and Italians Luciano Nicastro
and Marco Feroci of the BeppoSAX Gamma Ray Burst Team.
BeppoSAX is an Italian-Dutch satellite, launched late
last year, that detects gamma ray bursts and provides
precise sky positions to allow ground-based telescopes
to observe them.
Cosmic gamma ray bursts, occurring about once per
day, have been observed for some 30 years. However,
until this year, very little was known about them.
Even their distances from Earth were the subject of
great debate among astronomers.
This year, the rapid and improved positions provided
by the BeppoSAX satellite have allowed astronomers
to look for the bursts, or their optical/radio counterparts,
quickly with other instruments on the ground and in
space. As a result, two bursters, including the one
on May 8, have been detected optically. The VLA found
radio emission from the May 8 burster on May 13, and
the VLA and VLBA have been monitoring the object regularly
since then.
"Shri (Kulkarni) and I searched for radio counterparts
to gamma ray bursters for four years, but only when
BeppoSAX provided us with accurate positions were
we able to identify one," said Frail.
Gamma ray bursts, the causes of which still are unknown,
are short-lived phenomena. The May 8 burst lasted
only 15 seconds. However, X-ray, optical and radio
emission continues in an "afterglow." It is that afterglow
that is revealing details of the fireball created
by the initial explosion. That initial explosion released,
in 15 seconds, more energy than the sun will release
in its entire, 10-billion-year lifetime.
Optical studies of the May 8 object using the 10-meter
W.M. Keck Telescope in Hawaii indicated that it is
very distant, at least seven billion light-years away.
While the burst's afterglow showed a steady decline
in brightness at both optical and X-ray wavelengths,
that was not the case at radio wavelengths. The radio
emission rose and fell several times, and the relative
intensity at different radio wavelengths also changed.
While puzzling over this behavior, Frail and Kulkarni
learned that Jeremy Goodman of Princeton University
already was predicting this effect. Irregularities
in the extremely tenuous material between the stars
could cause fluctuations in the radio intensity as
seen from Earth. "As we collected more data, it became
clear to us that indeed the radio object 'twinkled,'"
Kulkarni said.
"Many amateur astronomers know that stars twinkle,
but planets don't," Frail explained. "This is because
the stars are so distant they appear as mere points,
and irregularities in the atmosphere cause them to
twinkle. Planets, on the other hand, are close enough
that they are not mere points, and their larger apparent
size squelches the twinkling. Over three months, we
saw the radio twinkling of the gamma ray burster slowly
cease. From this we could calculate both its apparent
size and the rate of its expansion."
"It is interesting to note that the irregularities
in the thin material between the stars enable us to
obtain information unobtainable any other way. Without
this effect, we would need a radio telescope array
the size of the sun to measure this object," said
Greg Taylor, who led the effort to observe the object
with the VLBA.
Using the ultra-sharp "vision" of the continent-wide
VLBA, the astronomers have pinpointed the burster's
position in the sky with extremely high precision
-- less than a thousandth of a second of arc. In the
three months of their observations, they have noted
that this position has not changed measurably, thus
strengthening the case for the object's being at a
great distance.
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.
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