Scientists Simulate Gamma Ray Fireball

CONTACT: Lia Unrau
PHONE:
(713) 348-6778
EMAIL: unrau@rice.edu

Graphics

SCIENTISTS
SIMULATE GAMMA RAY FIREBALL

Simulation Leads Way for
Creating Gamma Ray Burst Fireball in the Lab

With the goal of understanding how gamma ray bursts
are formed, astrophysicists have performed a computer simulation of what the
fireball hypothesized to exist in gamma-ray bursts would look like in the
laboratory setting.

The findings are being presented today by Edison
Liang of Rice University in Houston, and Scott Wilks of Lawrence Livermore
National Laboratory in Livermore, Calif., at the American Astronomical Society
meeting in Rochester, N.Y. Their paper is titled “Generating Relativistic and
Pair Plasmas with Ultraintense Lasers.”

By aiming ultraintense lasers at opposite sides of a
thin gold target, the researchers’ calculations show that much of the laser
energy is converted into positrons and electrons, or anti-matter and matter. As
soon as the lasers are turned off, the anti-matter and matter will explode
outward from the center at extremely high speeds. Their simulation shows clearly
that the electrons and positrons will leave the gold target material behind and
create a spherical shell, known as a pair corona. The speeds at which they
travel will be relativistic, or on the order of the speed of light.

“Normally the accelerated electrons will try to pull
out the gold ions, but here they are pulling out the positrons instead, which
are much lighter and more mobile,” Wilks said.

Their results indicate that in a physical experiment,
the positrons and electrons will indeed easily outrun the gold
material.

This is relevant, Liang said, because “this numerical
simulation suggests that indeed the electrons and positrons would stream out at
relativistic speeds, with energy as much as twenty times their rest mass,
leaving behind the gold ions. Therefore, you would be able to obtain what we
call a clean pair corona, of almost pure electrons and positrons. And that is an
analog of a gamma ray burst fireball.”

Once their simulations are confirmed with
experiments, they hope to demonstrate in the laboratory what happens when two
fireballs collide. “The interaction would most likely produce the kind of
physics that we anticipate produces the gamma ray bursts we observe,” Liang
said.

Liang and Wilks’ numerical simulations are proposed
to be tested with physical experiments in about two to three years, when the
National Ignition Facility, now under construction at Lawrence Livermore
National Lab, is ready to begin some experiments.

Conventional theory is that some kind of black hole
spews out fireballs in the form of spherical shells or jets, one after another.
When a later fireball is going a little faster than earlier ones, they will
collide, and it is hypothesized that that is when the gamma rays are produced.

“Obviously, these are computer simulations,” Liang
said of the findings presented today. “The key thing that gives us confidence
that the new simulation is probably correct is that our previous simulation of
electron-positron pair production using ultraintense lasers, published in Physical Review Letters two years ago, has been basically confirmed by
experiments,” Liang said. “And that simulation was done with the same
technology, the same numerical code.”

Liang and Wilks’ research was supported by the
Lawrence Livermore National Laboratory and NASA.

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Graphics

A
Phase Space Plot of Momentum Versus Position

An
Artist’s Conception of an Ultraintense Laser Hitting a Gold Target

Creating
a Pair Fireball

Contacts

Dr. Edison Liang, professor of
space physics and astronomy, Rice University, 713-348-3524, liang@rice.edu

Dr. Scott Wilks, physicist, Lawrence Livermore
National Laboratory, 925-422-2974, wilks1@llnl.gov