DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

gamma-ray burst, black hole accretion disks
light-years distant. At this distance, the energy
released is similar to the rest-mass energy of the
sun. Afterglow features match many of the theoreticalpredictionsofanexpanding“fireball,”
but
the source mechanism itself remains unknown.
Other transients, seemingly similar and found
with instrumentation designed to study gamma
ray bursts, have in recent years been confirmed
as distinctly separate phenomena, coming from
neutron stars located in this galaxy or its Magellanic
cloud satellite galaxies. See soft gamma
repeaters, March 5th event.
gamma-ray burst, black hole accretion disks
Under the gamma-ray burst “fireball” paradigm
(see gamma-ray bursts, fireball), any gammaray
burst engine must produce high energies
without ejecting too much baryonic matter. A
class of models, all of which produce rapidly accreting
tori around black holes, provide natural
explanations for the high energies, but low ejecta
mass. The gravitational potential energy is converted
to energy in a pair/plasma fireball either
from the neutrino annihilation or magnetic field
energy mechanisms, both of which require an
asymmetry in the mass accretion. The energy
is deposited along the disk rotation axis, producing
a beamed jet. This beaming lessens the
burst energy requirements (most papers quote
gamma-ray burst energies assuming isotropic
explosions) and avoids excessive baryon contamination.
This class of models includes mergers
of double neutron star systems, mergers of
black hole and neutron star binaries, mergers of
black hole and white dwarf binaries, collapsars,
and helium core mergers.
gamma-ray burst, classical Classical gamma-ray
bursts make up the bulk of the observed
gamma-ray bursts. They do not appear to repeat,
and have hard spectra (95% of energy emitted
by photons with energy greater than 50 keV).
However, beyond these characteristics, classical
gamma-ray bursts represent a very heterogeneous
set of objects. Burst durations range from
0.01 to 300 s, during which time the burst may
be chaotic, exhibiting many luminosity peaks,
or it may vary smoothly. They are distributed
isotropically and are thought to originate outside
of the galaxy (cosmological bursts). This
translates to burst energies in the range: ∼ 10 48
© 2001 by CRC Press LLC
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to 10 53 ergs, in some cases the most energetic
explosions in the universe since the Big Bang.
gamma-ray burst, classification Gammaray
bursts have been separated into two major
classes: soft gamma-ray repeaters and “classical”
gamma-ray bursts. Soft gamma-ray repeaters
repeat, have average photon energies of
30 to 50 keV, burst durations of ∼ 0.1 s, smooth
light-curves, and lie in the galactic plane. Classical
bursts do not seem to repeat, emit most of
their energy 50 keV, have a range of durations
(0.01 to 300 s) and light-curve profiles. Classical
bursts are distributed isotropically and are
thought to originate at cosmological distances
outside of the galaxy or galactic halo. Soft
gamma-ray repeaters are thought to be caused
by accretion, magnetic field readjustment, or
quakes in neutron stars in the galactic disk.
gamma-ray burst, cosmological mechanisms
Spectra of the optical counterpart of GRB970508
reveal many red-shifted (z = 0.835) absorption
lines, confirming that at least some gamma-ray
bursts are cosmological; that is, they occur outside
of the Milky Way. Cosmological models
provide a simple explanation for the isotropic
spatial distribution of gamma-ray bursts, but require
energies in excess of 10 51 ergs. Most cosmological
models rely upon massive accretion
events upon compact objects (e.g., the merger
of two neutron stars). Several mechanisms ultimately
produce accretion disks around black
holes where the gravitational energy released
can be more readily converted into relativistic
jets. These black-hole accretion disk models
provide an ideal geometry for facilitating the
potential energy conversion via the neutrino annihilation
or magnetic field mechanisms into
beamed gamma-ray burst jets.
gamma-ray burst, fireball The gamma-ray
burst fireball refers to a mechanism by which
the energy produced near the black hole or neutron
star source is converted into the observed
burst. Due to photon-photon scattering and electron
scattering opacities at the source of the
burst, the burst photons are initially trapped.
As the “fireball” expands adiabatically, the optical
depth decreases, but so does the temperature.
In the current scenario, the fireball does