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National Radio Astronomy Observatory
P.O. Box O
Socorro, NM 87801
http://www.nrao.edu

Contact:
Dave Finley, NRAO Public Information Officer, Socorro, NM
(505) 835-7302, dfinley{at}nrao.edu

EMBARGOED For Release: 10:00 a.m., CDT, May 28, 2003

Closest Gamma Ray Burst Providing Scientists With Crucial Test for
Burst Physics

The closest Gamma Ray Burst (GRB) yet known is providing astronomers
with a rare opportunity to gain information vital to understanding
these powerful cosmic explosions. Extremely precise radio-telescope
observations already have ruled out one proposed mechanism for the
bursts. 

"This is the closest and brightest GRB we've ever seen, and we can
use it to decipher the physics of how these bursts work," said Greg
Taylor of the National Radio Astronomy Observatory (NRAO) in Socorro,
NM. Taylor worked with Dale Frail, also of the NRAO, along with Prof.
Shri Kulkarni and graduate student Edo Berger of Caltech in studying
a GRB detected on March 29, 2003. The scientists presented their
findings to the American Astronomical Society's meeting in Nashville,
TN. 

Taylor and Frail used the National Science Foundation's (NSF) Very
Long Baseline Array (VLBA) and other radio telescopes to study the
burst, known as GRB 030329. In a series of observations from April 1
to May 19, they determined the size of the expanding "fireball" from
the burst and measured its position in the sky with great precision.

At a distance of about 2.6 billion light-years, GRB 030329 is hardly
next door. However, compared to other GRBs at typical distances of
8-10 billion light-years, it presents an easier target for study.

"We only expect to see one burst per decade this close," said Frail.

The precise measurement of the object's position allowed the
scientists to show that one theoretical model for GRBs can be ruled
out. This model, proposed in 2000, says that the radio-wave energy
emitted by the GRB comes from "cannonballs" of material shot from the
explosion at extremely high speeds. 

"The 'cannonball model' predicted that we should see the
radio-emitting object move across the sky by a specific amount. We
have not seen that motion," Taylor said.

The currently standard "fireball model" of GRBs says that the radio
emission comes from a rapidly-expanding shock wave. This model was
first proposed by Peter Meszaros, Bohdan Paczynski and Sir Martin
Rees, who won the American Astronomical Society's Bruno Rossi Prize
in 2000 for their work. In this standard model, as the shock wave
expands outward, the emission becomes fainter, but the center of the
observed emission does not change position. 

The cannonball model, however, proposes that the emission arises from
distinct concentrations of matter shot outward from the burst. As
they move farther from the burst, their motion should be detected as
a change in their position in the sky. On April 3, proponents of the
cannonball model predicted a specific amount of motion for GRB 030329
and suggested that the VLBA's sharp radio "vision" could detect the
motion and confirm their prediction. 

Instead, "our observations are consistent with no motion at all,"
Taylor said. "This is at odds with the cannonball model -- they made
a specific prediction based on their model and the observations do
not bear them out," he added. 

The scientists' direct measurement of the size of the GRB fireball
also will provide new insights into the physics behind the burst.

"By directly measuring the size and the expansion rate, we can start
putting some real limits on the physics involved," Taylor said.
First, he said, "We already can confirm that the fireball is
expanding at nearly the speed of light, as the standard model
predicts. Next, once our May observations are fully analyzed, we can
put limits on the energy of the burst and provide a test of the
standard model."

Taylor and Frail observed GRB 030329 with the VLBA on April 1 and
April 6. On April 22, they used the 100-meter radio telescope in
Effelsberg, Germany in addition to the VLBA. On May 19, they used the
VLBA, the Very Large Array (VLA) in New Mexico, the NSF's Robert C.
Byrd Green Bank Telescope in West Virginia, and the Effelsberg
telescope. 

In addition to gamma-ray and X-ray observations, visible light from
GRB 030329 was observed by 65 telescopes around the world. At its
brightest, the visible light from this burst was detectable with
moderate-sized amateur telescopes. 

Gamma Ray Bursts were first detected in 1967 by a satellite
monitoring compliance with the 1963 atmospheric nuclear test-ban
treaty. For three decades thereafter, astronomers were unable to
determine their distances from Earth, and thus were unable to begin
understanding the physics underlying the explosions. In 1997, the
first distance measurements were made to GRBs, and the NSF's Very
Large Array (VLA) detected the first radio emission from a GRB
afterglow.
 
Once scientists determined that GRBs originate in distant galaxies
and that they probably occur in regions of those galaxies where stars
are actively forming, some 200 proposed models for what causes GRBs
were reduced to a handful of viable models.

Most scientists now believe that GRBs arise from a violent explosion
that ends the life of a star much more massive than the Sun. Whereas
such an explosion as a typical supernova leaves a dense neutron star,
a GRB explosion leaves a black hole, a concentration of mass with
gravitational pull so strong that not even light can escape it.

The VLBA is a continent-wide system of ten radio- telescope antennas,
ranging from Hawaii in the west to the U.S. Virgin Islands in the
east, providing the greatest resolving power, or ability to see fine
detail, in astronomy. Dedicated in 1993, the VLBA is operated from
the NRAO's Array Operations Center in Socorro, New Mexico.

The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.

IMAGE CAPTION:
[http://www.nrao.edu/pr/2003/grb030329/grb030329.vlba.gif (93KB)]
VLBA IMAGE of GRB 030329. CREDIT: NRAO/AUI/NSF

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