June 23, 2003
UCSC scientist part of team decoding gamma-ray
By Christopher Watjen
Scientists have pieced together the key elements of a gamma-ray burst,
from star death to dramatic black hole birth, thanks to a March 29 explosion
considered the "Rosetta stone" of such bursts.
This computer simulation shows the beginning of a gamma-ray
burst. Above, the jet is shown 9 seconds after its creation at
the center of a Wolf Raye star by the newly formed, accreting
black hole within. Below, the jet is just erupting through the
surface of the Wolf Rayet star, which has a radius comparable
to that of the sun. Blue represents regions of low mass concentration,
red is denser, and yellow denser still. Note the blue and red
striations behind the head of the jet. These are bounded by internal
shocks. Images: Weiqun Zhang and Stan
|Additional images and animations
are available online.
The results are described in the June 19 issue of Nature, in
an article coauthored by Stan Woosley, professor and chair of astronomy
and astrophysics at the University of California, Santa Cruz.
The telling March 29 burst in the constellation Leo, one of the brightest
and closest on record, reveals for the first time that a gamma-ray burst
and a supernova--the two most energetic explosions known in the universe--occur
essentially simultaneously, a quick and powerful one-two punch.
The burst was detected by NASA's High-Energy Transient Explorer (HETE)
and observed in detail with the European Southern Observatory's Very
Large Telescope (VLT) at the Paranal Observatory in Chile.
"The March 29 burst changes everything," said Woosley. "With
this missing link established, we know for certain that at least some
gamma-ray bursts are produced when black holes, or perhaps very unusual
neutron stars, are born inside massive stars. We can apply this knowledge
to other burst observations."
The research team said that just as the Rosetta stone helped us understand
an ancient language, this burst will serve as a tool to decode other
Woosley and his graduate student, Weiqun Zhang, created computer simulations
of a gamma-ray burst using one of the fastest unclassified computers
in the world, at Lawrence Berkeley National Laboratory. Using 128 computer
processors simultaneously, Woosley said the simulations took about two
weeks--or about 25,000 processor hours. Woosley is director of the Center
for Supernova Research, funded by the Department of Energy's Scientific
Discovery through Advanced Computing (SciDAC) program. The other partner
institutions are the University of Arizona and the Los Alamos and Lawrence
Livermore National Laboratories.
A supernova is the explosion of a star at least eight times as massive
as the Sun.
When such stars deplete their nuclear fuel, they no longer have the
energy to support their mass. Their cores implode, forming either a
neutron star or (if there is enough mass) a black hole. The surface
layers of the star blast outward, becoming a billion times as luminous
as the Sun.
Scientists have suspected gamma-ray bursts and supernovae were related,
but they have had little observational evidence, until March 29.
"We've been waiting for this one for a long, long time,"
said Jens Hjorth, University of Copenhagen, lead author of the Nature
article, one of three related "letters" appearing in the issue.
"The March 29 burst contains all the missing information. It was
created through the core collapse of a massive star."
The research team said that the Rosetta stone burst also provides a
lower limit on how energetic gamma-ray bursts truly are and rules out
most theories concerning the origin of "long bursts," lasting
longer than two seconds.
Gamma-ray bursts temporarily outshine the entire universe in gamma-ray
light, packing the energy of over 100 million billion Suns.
"It is as bright as if two thousand Earth masses were abruptly
turned to energy in the form of gamma rays," said Woosley. "Any
life within the path of the burst inside of several hundred light-years
would have been obliterated."
Yet these explosions are fleeting--lasting only seconds to minutes--and
occur randomly from all directions in the sky, making them difficult
GRB 030329, named after its detection date, occurred relatively close,
approximately 2 billion light-years away (at redshift 0.1685). The burst
lasted over 30 seconds. ("Short bursts" are less than 2 seconds
long.) GRB 030329 is among the 0.2 percent brightest bursts ever recorded.
Its afterglow lingered for weeks in lower-energy X-ray and visible light.
With the VLT, Hjorth and his colleagues uncovered evidence in the afterglow
of a massive, rapidly expanding supernova shell, called a hypernova,
at the same position and created at the same time as the afterglow.
The following scenario emerged:
Thousands of years prior to this explosion, a very massive star, running
out of fuel, let loose much of its outer envelope, transforming itself
into a bluish Wolf-Rayet star. The Wolf-Rayet star--containing about
10 solar masses worth of helium, oxygen, and heavier elements--rapidly
depleted its fuel, triggering the Type Ic supernova/gamma-ray burst
event. The core collapsed, without the star's outer part knowing. A
black hole formed inside surrounded by a disk of accreting matter, and,
within a few seconds, launched a jet of matter away from the black hole
that ultimately made the gamma-ray burst.
The jet passed through the outer shell of the star and, in conjunction
with vigorous winds of newly forged radioactive nickel-56 blowing off
the disk inside, shattered the star. This shattering represents the
Meanwhile, collisions among pieces of the jet moving at different velocities,
all very close to light speed, created the gamma-ray burst. This "collapsar"
model, introduced by Woosley in 1993, best explains the observation
of GRB 030329, as opposed to the "supranova" and "merging
neutron star" models.
In previous gamma-ray bursts, scientists had found evidence of iron
in the afterglow light, a signature of a star explosion. Also, the location
of a supernova occurring in 1998, named SN1998bw, appeared to be in
the same vicinity as a gamma-ray burst. The data was inconclusive, however,
and many scientists remained skeptical of the association.
"Supernova 1998bw whetted our appetite," said coauthor Chryssa
Kouveliotou of the NASA Marshall Space Flight Center in Huntsville,
Alabama. "But it took five more years before we could confidently
say we found the smoking gun that nailed the association between gamma-ray
bursts and supernovae."
"This does not mean that the gamma-ray burst mystery is solved,"
"We are confident that long bursts involve a core collapse, probably
creating a black hole. We have convinced most skeptics. We cannot reach
any conclusion yet, however, on what causes short gamma-ray bursts."
Short bursts might be caused by neutron star mergers. A NASA-led international
satellite named Swift, to be launched in January 2004, will "swiftly"
locate gamma-ray bursts and may capture short-burst afterglows, which
have yet to be detected.
The VLT is the world's most advanced optical telescope, comprising
four 8.2-meter reflecting Unit Telescopes and, in the future, four moving
1.8-meter Auxiliary Telescopes for interferometry. HETE was built by
MIT as a mission of opportunity under the NASA Explorer Program, with
collaboration among U.S. universities, Los Alamos National Laboratory,
and scientists and organizations in Brazil, France, India, Italy, and
Louise Donahue contributed
to this story.
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