Gamma-Ray Bursts, X-Ray Flashes, and Supernovae Not As Different As They Appear

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Caltech

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Robert Tindol, (626) 395-3631,

November 12, 2003

Gamma-Ray Bursts, X-Ray Flashes, and Supernovae Not As Different As They Appear

PASADENA, Calif. -- For the past several decades, astrophysicists have been
puzzling over the origin of powerful but seemingly different explosions that
light up the cosmos several times a day. A new study this week demonstrates that
all three flavors of these cosmic explosions -- gamma-ray bursts, X-ray flashes,
and certain supernovae of type Ic -- are in fact connected by their common
explosive energy, suggesting that a single type of phenomenon, the explosion of
a massive star, is the culprit. The main difference between them is the "escape
route" used by the energy as it flees from the dying star and its newly born
black hole.

In the November 13 issue of the journal Nature, Caltech graduate student Edo
Berger and an international group of colleagues report that cosmic explosions
have pretty much the same total energy, but this energy is divided up
differently between fast and slow jets in each explosion. This insight was made
possible by radio observations, carried out at the National Radio Astronomy
Observatory's Very Large Array (VLA), and Caltech's Owens Valley Radio
Observatory, of a gamma-ray burst that was localized by NASA's High Energy
Transient Explorer (HETE) satellite on March 29 of this year.

The burst, which at 2.6 billion light-years is the closest classical gamma-ray
burst ever detected, allowed Berger and the other team members to obtain
unprecedented detail about the jets shooting out from the dying star. The burst
was in the constellation Leo.

"By monitoring all the escape routes, we realized that the gamma rays were just
a small part of the story for this burst," Berger says, referring to the nested
jet of the burst of March 29, which had a thin core of weak gamma rays
surrounded by a slow and massive envelope that produced copious radio waves.

"This stumped me," Berger adds, "because gamma-ray bursts are supposed to
produce mainly gamma rays, not radio waves!"

Gamma-ray bursts, first detected accidentally decades ago by military satellites
watching for nuclear tests on Earth and in space, occur about once a day. Until
now it was generally assumed that the explosions are so titanic that the
accelerated particles rushing out in antipodal jets always give off prodigious
amounts of gamma radiation, sometimes for hundreds of seconds. On the other
hand, the more numerous supernovae of type Ic in our local part of the universe
seem to be weaker explosions that produce only slow particles. X-ray flashes
were thought to occupy the middle ground.

"The insight gained from the burst of March 29 prompted us to examine previously
studied cosmic explosions," says Berger. "In all cases we found that the total
energy of the explosion is the same. This means that cosmic explosions are
beasts with different faces but the same body."

According to Shri Kulkarni, MacArthur Professor of Astronomy and Planetary
Science at Caltech and Berger's thesis supervisor, these findings are
significant because they suggest that many more explosions may go undetected.
"By relying on gamma rays or X rays to tell us when an explosion is taking
place, we may be exposing only the tip of the cosmic explosion iceberg."

The mystery we need to confront at this point, Kulkarni adds, is why the energy
in some explosions chooses a different escape route than in others.

At any rate, adds Dale Frail, an astronomer at the VLA and coauthor of the
Nature manuscript, astrophysicists will almost certainly make progress in the
near future. In a few months NASA will launch a gamma-ray detecting satellite
known as Swift, which is expected to localize about 100 gamma-ray bursts each
year. Even more importantly, the new satellite will relay very accurate
positions of the bursts within one or two minutes of initial detection.

The article appearing in Nature is titled "A Common Origin for Cosmic Explosions
Inferred from Calorimetry of GRB 030329." In addition to Berger, the lead
author, and Kulkarni and Frail, the other authors are Guy Pooley, of Cambridge
University's Mullard Radio Astronomy Observatory; Vince McIntyre and Robin Wark,
both of the Australia Telescope National Facility; Re'em Sari, associate
professor of astrophysics and planetary science at Caltech; Derek Fox, a
postdoctoral scholar in astronomy at Caltech; Alicia Soderberg, a graduate
student in astrophysics at Caltech; Sarah Yost, a postdoctoral scholar in
physics at Caltech; and Paul Price, a postdoctoral scholar at the University of
Hawaii's Institute for Astronomy.

*****

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November 12, 2003

Despite Appearances, Cosmic Explosions Have Common Origin, Astronomers Discover

A Fourth of July fireworks display features bright explosions that light the sky
with different colors, yet all have the same cause. They just put their
explosive energy into different colors of light. Similarly, astronomers have
discovered, a variety of bright cosmic explosions all have the same origin and
the same amount of total energy.

This is the conclusion of an international team of astronomers that used the
National Science Foundation's Very Large Array (VLA) radio telescope to study
the closest known gamma-ray burst earlier this year.

"For some reason we don't yet understand, these explosions put greatly varying
percentages of their explosive energy into the gamma-ray portion of their
output," said Dale Frail, of the National Radio Astronomy Observatory (NRAO) in
Socorro, NM. That means, he said, that both strong and weak gamma-ray bursts,
along with X-ray flashes, which emit almost no gamma rays, are just different
forms of the same cosmic beast. The research team reported their results in the
November 13 issue of the scientific journal Nature.

The scientists trained the VLA on a gamma-ray burst discovered using NASA's
HETE-2 satellite last March 29. This burst, dubbed GRB 030329, was the closest
such burst yet seen, about 2.6 billion light-years from Earth. Because of this
relative proximity, the burst was bright, with visible light from its explosion
reaching a level that could be seen in amateur telescopes. As the burst faded,
astronomers noted an underlying distinctive signature of a supernova explosion,
confirming that the event was associated with the death of a massive star.

Since 1999, astronomers have known that the strong outbursts of gamma rays,
X-rays, visible light and radio waves from these bursts form beams, like those
from a flashlight, rather than spreading in all directions, like light from a
bare bulb. The surprising result from the VLA studies of GRB 030329 is that
there are two beams, not one. The scientists found that the gamma rays and the
early visible-light and X-ray emission were coming from a narrow beam, while the
radio waves and later visible-light emission came from another, wider beam.

"The strange thing is that some explosions seem to put most of their energy into
the narrow beam, while others put most or nearly all their energy into the wider
beam," Frail said. "This is telling us something very fundamental about the
inner workings that drive these explosions," Frail added.

The mechanism producing these explosions is what scientists call a collapsar,
which occurs when a giant star collapses of its own weight at the end of its
normal, nuclear fusion-powered lifetime. In an ordinary supernova, such a
collapse produces a neutron star. A collapsar, however, marks the death of a
more-massive star and results in a black hole, a concentration of mass so dense
that not even light can escape it.

After the black hole forms, its powerful gravitational pull sucks the star's
remaining material toward it. This material forms a spinning disk around the
black hole that lasts only a few seconds. During that time, the disk ejects
material outward from its poles. A jet of material moving at nearly the speed of
light emits gamma rays; slower material emits radio waves and visible light.

"Despite the differences in how much energy comes out in gamma rays, all these
things seem to be caused by the same basic mechanism," said Edo Berger, a
graduate student at Caltech and lead author of the Nature paper. "Our
observations now give the data that will help us understand what causes the
differences," he added.

"It was astounding to suddenly realize that these apparently very different
cosmic beasts all are really the same thing," said Berger.

The next job, Frail said, is to learn if there are, in fact, two jets, or a
single jet in which the central part encounters less resistance and thus can
move outward at greater speeds.

Frail pointed out that the radio observations alone had the ability to show the
total energy output of the burst, thus providing the breakthrough in
understanding the common thread among the different types of explosions. "The
key fact is that the optical, X-Ray and gamma-ray telescopes missed 90 percent
of the energy put out by this burst," Frail added.

"As the VLA Expansion Project progresses and the sensitivity of the VLA improves
in the coming years, it will become an even more important tool in unravelling
this mystery," Frail said.

"The exciting part of this new discovery is that explosions that we once thought
were quite different now appear to all have a common origin," Frail concluded.
"That insight, of course, gives us the new challenge of explaining how a single
mechanism can make itself look so different," he added.

In addition to Berger and Frail, the other authors of the paper are Professor
Shri Kulkarni of Caltech; Guy Pooley of Cambridge University's Mullard Radio
Astronomy Observatory; Vince McIntyre and Robin Wark, both of the Australia
Telescope National Facility; Re'em Sari, associate professor of astrophysics and
planetary science at Caltech; Derek Fox, a postdoctoral scholar in astronomy at
Caltech; Alicia Soderberg, a graduate student in astrophysics at Caltech; Sarah
Yost, a graduate student in physics at Caltech; and Paul Price, a postdoctoral
scholar at the University of Hawaii's Institute for Astronomy. Berger and
Soderberg earlier worked on gamma-ray-burst studies as summer students at NRAO.

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/grbtwinjet/grb.twinjet.gif (304KB)]
Artist's Conception of Twin Jets in Energetic Cosmic Explosion. CREDIT: Dana
Berry, SkyWorks Digital