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Gamma Ray Burst
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October 13,
1998

Gamma-ray Bursters cross the 'Line of Death'

Fireballs and gamma-ray bursts are not the same thing

Cosmic gamma-ray bursts have been called
the greatest mystery of modern astronomy. They are powerful blasts
of gamma- and X-radiation that come from all parts of the sky, but
never from the same direction twice. Space satellites indicate that
Earth is illuminated by 2 to 3 bursts every day.

What are
they? No one is certain. Until recently we didn't even know if they
came from the neighborhood of our own solar system or perhaps from
as far away as the edge of the universe. The first vital clues began
to emerge in 1997 when astronomers detected an optical counterpart
to a gamma-ray burst. In February 1997 the BeppoSAX X-ray astronomy
satellite pinpointed the position of a burst in Orion to within a
few arcminutes. That allowed astronomers to photograph the burst,
and what they saw surprised them. They detected a rapidly fading
star, probably the aftermath of a gigantic explosion, next to a
faint amorphous blob believed to be a very distant galaxy.

The Burst and
Transient Source Experiment ( BATSE
) aboard the Compton Gamma Ray Observatory, pictured above, has
recorded over 2000 cosmic gamma-ray bursts since it began operations
in 1991.

This image from the Hubble Space Telescope shows
the optical afterglow from a gamma-ray burst detected in February
1997. The bright spot is thought to be an expanding fireball, and
the weak diffuse emission (below and to the right) may be the
distant host galaxy.

Since then seven more optical
counterparts have been discovered. A recent discovery makes
gamma-ray bursts seem more fantastic than ever. Shri Kulkarni of
Caltech and his colleagues found that a gamma-ray burst recorded in
December 1997 came from a faint galaxy with a redshift of 3.4. That
means that the burst originated over 12 billion light years away.
Kulkarni noted that "The energy released by this burst in its first
few seconds staggers the imagination." Indeed, it was one of the
biggest explosions since the Big Bang itself.

Now that we
know where gamma-ray bursts come from -- very far away --
the next daunting task is to understand what causes them.
In the absence of much hard data theorists have proposed a multitude
of possible scenarios, from super-supernovae to mutually
annihilating neutron stars. It is widely thought that the x-ray,
optical, and radio afterglows might provide some clues.

The light curves of the few known
optical and X-ray counterparts are consistent with that of an
expanding fireball that is glowing because of a "Synchrotron Shock".
The basic idea is that a tremendous explosion ejects a shock wave of
material that accelerates charged particles, like electrons and
protons, to velocities near the speed of light. Dr. Robert Preece, a
gamma-ray astrophysicist at the University of Alabama in Huntsville,
likened the shock wave to a wave on the beach. "A shock forms when
the wave crest starts to fall over, and scud from the wave shoots
out ahead."

A synchrotron shock wave can be visualized as an
ocean wave. A shock forms when the wave crest starts to fall over,
and scud from the wave shoots out ahead. In the cosmic shock wave,
the 'scud' is composed of charged particles that give rise to
synchrotron radiation.

In the cosmic shock wave, the 'scud' is electrons
and protons. They accelerate ahead of the wave and spiral around
magnetic fields lines, producing a form of radiation called
synchrotron emission. Synchrotron emission is seen all the time here
on Earth as a blue glow in particle accelerators, and radio
astronomers detect it coming from the Milky
Way.

Fortunately, the Synchrotron Shock Model makes a
testable prediction. The spectrum of a typical gamma-ray burst looks
like the plot at the right. At lower energies, i.e., less than a few
hundred keV (kilo-electron volts), the spectrum is fairly flat. A
nearly horizontal line fits that part of the spectrum fairly well.
At higher energies the spectrum is steeper. The Synchrotron Shock
Model predicts that the slope of the line that fits the lower energy
part of the spectrum cannot be greater than -2/3.

In a
recently published edition of the Astrophysical Journal Letters
Rob Preece and his collaborators from the University of Alabama
examined over 100 bright bursts collected by the BATSE instrument on
the Compton Gamma Ray Observatory and measured the slopes of their
low-energy spectra. The figure at left shows their data.

They plotted the slope of the low energy part of
the spectra (vertical axis) vs. the peak burst energy (horizontal
axis). The red line is the so-called "Line of Death", corresponding
to a spectral slope greater than -2/3. If a data point falls above
the line, that gamma-ray burst cannot have been caused by a
synchrotron shock, and thus the Synchrotron Shock Model is "dead"
for that burst. Preece et al. found that 44% of the bursts fell
above the Line of Death. If we assume that all gamma-ray bursts are
caused by the same thing, this means that none can be due to a
synchrotron shock.

Thanks to their work we now know another
thing that gamma-ray bursts are not . They can't be caused
by a synchrotron shock. Interestingly though, there is strengthening
evidence that the optical counterparts, the glow from fireballs that
appear to be the aftermath of gamma-ray bursts in distant galaxies,
are caused by synchrotron shock waves. Whatever makes the
fireball glow is apparently different from the mechanism that makes
gamma-ray bursts. It's yet another mystery in the fantastic saga of
gamma-ray bursters.

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