DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

not become optically thin until much of the internal
energy has been converted into kinetic energy
through expansion. The observed gammaray
burst spectrum is then produced by internal
shocks in the fireball or by shocks created as the
fireball sweeps up the interstellar medium.
gamma-ray burst, galactic mechanisms
Prior to the launch of the Burst and Transient
Source Experiment (BATSE), galactic models
were the “favored” mechanisms for gamma-ray
bursts. The bulk of these models involve sudden
accretion events onto neutron stars or some
sort of glitch in the neutron star (e.g., neutron
star quakes). However, the data from BATSE
revealed that the bursts are isotropically distributed
in the sky, limiting galactic models to
those that occur in the galactic halo. The advantage
of such mechanisms is that the energy
requirements for galactic explosions is nearly
8 orders of magnitude lower than their cosmological
counterparts (∼ 10 43 ergs). Recent observations
of the optical afterglow of gamma-ray
bursts, specifically the detection of redshifted
lines in GRB970508, place at least some of the
gamma-ray bursts at cosmological redshifts.
gamma-ray burst, GRB 970508 The May
8, 1997 gamma-ray burst is the first gammaray
burst where a reliable optical counterpart revealed
identifiable metal absorption lines. The
lines are probably caused by some intervening
material between our galaxy and the gammaray
burst source. A redshift of the absorbing
medium of z = 0.835 was inferred from these
8 lines, setting the minimum redshift of the
gamma-ray burst (Metzger et al., 1997). Other
evidence indicates that the absorbing material
was part of the host galaxy of the gamma-ray
burst, placing the gamma-ray burst at a redshift
z= 0.835. This gamma-ray burst provides indisputable
evidence that at least some gammaray
bursts are cosmological.
gamma-ray burst, hypernova The hypothetical
explosion produced by a collapsar, the
collapse of a rotating massive star into a black
hole, which would produce a very large γ -ray
burst. See gamma-ray burst models, collapsar.
© 2001 by CRC Press LLC
gamma-ray burst models, collapsar
gamma-ray burst, magnetic fields The generation
and stretching of magnetic field lines
have been proposed as mechanisms to convert
the energy of material accreting onto a black
hole in active galactic nuclei and gamma-ray
bursts alike. The “standard” mechanism is
that described by Blandford and Znajek (1977)
which uses magnetic field interactions in the
disk to extract the rotational energy of the
black hole. Other mechanisms exist which extract
the potential energy of the accreting matter.
Gamma-ray bursts require magnetic field
strengths in excess of ∼ 10 15 Gauss, which is
roughly 10% of the disk equipartion energy.
gamma-ray burst, mechanisms Over 100
proposed distinct mechanisms for classical
gamma-ray bursts exist which can be grouped
roughly into three categories based on their location:
solar neighborhood GRBs, galactic GRBs,
and cosmological GRBs. Solar neighborhood
GRB mechanisms are the least likely and calculations
of the proposed mechanisms do not
match the observations. Galactic models have
the advantage that they require much less energy
than cosmological models. However, the
isotropic distribution of bursts require that these
models be in the galactic halo. In addition, absorption
lines in the spectra of the optical counterparts
of GRBs, namely GRB970508, indicate
that some bursts must be cosmological. Because
of this evidence, cosmological models are the favored
class of models, despite their high energy
requirements (up to 10 53 ergs).
The major constraint of any mechanism
is that it must produce sufficient energy (
10 51 ergs for an isotropic cosmological burst)
with relatively little contamination from
baryons. The low mass in the ejecta is required
to achieve the high relativistic velocities
(Ɣ = 1 + Eburst/Mejecta). Beaming of the
burst reduces both the energy requirement (the
energies generally quoted in papers assume an
isotropic explosion) and limit the baryonic contamination.
This beaming is predicted by most
of the viable gamma-ray burst models.
gamma-ray burst models, collapsar The
cores of stars with masses above ∼ 10M⊙
overcome electron degeneracy pressure and collapse.
For stars with masses 25 to 50M⊙, this
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