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  ![](media/image1.jpeg){width="1.4791666666666667in"   ***Advanced Scientific\                 ![](media/image2.png){width="0.6666666666666666in"
  height="0.4791666666666667in"}                        Computing Research***                   height="0.4930555555555556in"}

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> **Simulation Matches Historic Gamma-Ray Burst (2003)**

## Stan Woosley and Weiqun Zhang, University of California, Santa Cruz

***The collapsar model of the origin of long gamma-ray bursts (GRBs) was
already being simulated on NERSC computers when an unusually bright and
close GRB conclusively established that at least some long GRBs come
from supernovae, and it singled out the collapsar model as the one that
best fits the data collected so far.***

After three decades of scientific head-scratching, the origins of at
least some gamma-ray bursts (GRBs) are being revealed, thanks to a new
generation of orbiting detectors, fast responses from ground-based
robotic telescopes, and a new generation of computers and astrophysics
software.

A GRB detected on March 29, 2003 has provided enough information to
eliminate all but one of the theoretical explanations of its origin.
Computational simulations based on that model were already being
developed at the NERSC Center at when the discovery was made.

Gamma radiation from outer space is blocked by the Earth's atmosphere.
But if it were not, and if we could see gamma rays with our eyes, about
once a day, somewhere in the world, people would see a spectacular
flash, a hundred million times brighter than a supernova. These
gamma-ray bursts, first discovered in the 1960s by satellites looking
for violations of the Nuclear Test Ban Treaty, are the most energetic
events in the Universe, but they are also among the most elusive. GRBs
appear randomly from every direction. They do not repeat themselves. And
they last from only a few milliseconds to a few minutes---astronomers
consider a GRB long if it lasts longer than 2 seconds.

With data so hard to pin down, scientists for a long time could only
speculate about what causes GRBs. Speculations ranged from the
intriguing (comet/anti-comet annihilation) to the far-fetched
(interstellar warfare). By 1993, 135 different theories on the origin of
GRBs had been published in scientific journals.

But in recent years, three theoretical models emerged as
frontrunners---one involving colliding neutron stars, and the other two
involving supernovae, the "supranova" and "collapsar" models.

In the collapsar model (introduced in 1993 by Stan Woosley, Professor
and Chair of Astronomy and Astrophysics at the University of California,
Santa Cruz), the iron core of an aging star runs out of fuel for nuclear
fusion and collapses of its own weight, creating a black hole or a dense
neutron star. Material trying to fall onto this object forms a hot
swirling disk and a narrow jet, which shoots out of the star in less
than ten seconds at nearly the speed of light. When this jet erupts into
interstellar space, it creates a "fireball" of gamma rays---the highest
energy, shortest wavelength form of electromagnetic radiation. The rest
of the star explodes as a supernova, but that event is eclipsed by the
brighter GRB.

In the late 1990s, GRB research took a great leap forward when a new
generation of orbiting detectors were launched, including the
Italian-Dutch satellite BeppoSAX and NASA's High-Energy Transient
Explorer (HETE). These satellites were designed to provide quick
notification of GRB discoveries for immediate follow-up observations by
ground-based instruments. In 1997 astronomers discovered that long GRBs
leave an afterglow of lower-energy light, such as X rays or visible
light, that may linger for months, allowing researchers to pinpoint
where the GRB originated and providing important clues about the event
that produced it. The presence of iron in the afterglow light strongly
suggested star explosions. In 1998, the supernova theories got an
additional boost when a GRB and a supernova appeared in the same
vicinity at roughly the same time; but the data was inconclusive and
some scientists remained skeptical about the connection. Several similar
suggestive but inconclusive events in subsequent years divided
astronomers into two camps on the issue of associating GRBs with
supernovae.

The skepticism was dispelled on March 29, 2003, when HETE detected an
unusually bright and close

+-----------------------------------+-----------------------------------+
| ![](media/image3.                 | ![](media/image4.                 |
| png){width="3.1618055555555555in" | png){width="3.1618055555555555in" |
| height="1.8993055555555556in"}    | height="1.8993055555555556in"}    |
+-----------------------------------+-----------------------------------+
| *Fig. 1. This image from a        | > *Fig. 2. This image from a      |
| computer simulation of the        | > computer simulation shows the   |
| beginning of a gamma-ray burst    | > distribution of relativistic    |
| shows the jet 9 seconds after its | > particles (moving near light    |
| creation at the center of a       | > speed) in the jet as it breaks  |
| Wolf-Rayet star by the newly      | > out of the star. Yellow and     |
| formed, accreting black hole      | > orange are very high energy and |
| within. The jet is now just       | > will ultimately make a          |
| erupting through the surface of   | > gamma-ray burst, but only for   |
| the star, which has a radius      | > an observer looking along the   |
| comparable to that of the sun.    | > jet (± about 5 degrees). Note   |
| Blue represents regions of low    | > also the presence of some small |
| mass concentration, red is        | > amounts of energy in mildly     |
| denser, and yellow denser still.  | > relativistic matter (blue) at   |
| Note the blue and red striations  | > larger angles off the jet.      |
| behind the head of the jet. These | > These will produce X-ray        |
| are bounded by internal shocks.*  | > flashes that may be seen much   |
|                                   | > more frequently.*               |
+-----------------------------------+-----------------------------------+

GRB---only 2.6 billion light years from Earth instead of the typical 10
billion. The discovery triggered a swarm of observations that found the
unmistakable spectral signature of a supernova in the afterglow, as
reported in the June 19, 2003 issue of Nature. This event, named
GRB030329 after its detection date, was dubbed the "Rosetta stone" of
GRBs because it conclusively established that at least some long GRBs
come from supernovae, and it singled out the collapsar model as the one
that best fits the data collected so far. The March 29 burst's afterglow
was so bright that it allowed astronomers to study the event in
unprecedented detail; they even joked about its casting shadows. The
Hubble Space Telescope was able to study the afterglow for another year,
and radio telescopes were able to track it longer.

The first 3D computational simulations of jet formation and breakout in
the collapsar model (Figs. 1 and 2) were already being conducted on
NERSC's Seaborg supercomputer by Woosley and Weiqun Zhang, a Ph.D.
candidate at UC Santa Cruz, under the sponsorship of the DOE Office of
Science's SciDAC Supernova Science Center, one of two SciDAC (Scientific
Discovery through Advanced Computing) programs focusing on developing
new computational methods for understanding supernovae. The goal of the
Supernova Science Center is to discover the explosion mechanism of
supernovae through numerical simulation.

Zhang, Woosley, and colleagues had been modeling jet formation after
core collapse in both two and three dimensions for a few years when
observations from GRB030329 validated their work and made it newsworthy.
"We weren't utterly surprised," Woosley said, "because the evidence
associating GRBs with supernovae had been accumulating for several
years. But we weren't totally confident, either. When the light spectrum
from the March 29 burst confirmed that it came from a supernova, that
felt good."

Woosley emphasized, however, that "the data is still way ahead of the
theory." The computational simulations are still incomplete, and the
resolution needs to be improved. Upcoming work will include
higher-resolution 3D studies of jet stability---which will help
scientists interpret the differences in GRB observations---as well as
simulating the full star explosion that accompanies the GRB and the core
collapse that precedes it.

**Publications:**

J. Hjorth, J Sollerman, P. Møller, J. P. U. Fynbo, S. E. Woosley, et
al., "A very energetic supernova associated with the -ray burst of 29
March 2003," Nature **423**, 847 (2003).

**Funding:**

DOE Office of Nuclear Physics, SciDAC