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Gamma-ray bursts
Since their discovery in 1973, Gamma-Ray Bursts (GRBs) have been one
of the most intriguing mysteries in modern astronomy. Observations
made using BATSE on CGRO and the detectors on BeppoSAX have revolutionized
our understanding. The isotropic distribution of GRBs detected by
BATSE was interpreted as evidence of extragalactic origin (Megan et
al. 1992, Nature 355, 143). The conclusive breakthrough came with the
discovery of a fading X-ray (Costa et al. 1997, Nature 387,783) and
optical afterglow (Van Paradijs, 1997, Nature 368, 686) from GRB
970228 (Fig. 9). The few arcminute error circle from the BeppoSAX
afterglow made possible the identification with a 25th
magnitude
dwarf galaxy at z ~0.835 -- confirming the cosmological nature
of GRBs. More than twenty GRB afterglows have been followed by BeppoSAX,
RXTE and ASCA, most of which are available at the HEASARC
archive. The upcoming Swift and HETE missions will gather up to a
thousand redshift measurements using afterglows and will most
likely identify many different classes of GRB. The BATSE archive of
several thousand bursts will be a valuable resource to retroactively
classify and study the properties of GRBs.
The BATSE archive of gamma-ray burst data has been a rich
resource for research. Once the public archive was started, with the
release of the BATSE 1B catalog of 260 bursts, the data were widely used
to study the isotropy and homogeneity of bursts (e.g. Quashnock & Lamb
1993, MNRAS 265, L45). Currently the BATSE archive contains over 2500
bursts, and is large enough for study of different subclasses of bursts,
characterized by apparent brightness, duration, spectra, or the shape of
the burst profile. Archival studies of the distribution and the spectral
and temporal evolution of bursts are numerous and ongoing. Selected
recent examples are discussed below.
BeppoSAX observations of the first X-ray afterglow
from GRB 970228 at 8 hr and 3 days after the GRB detection. These
data are part of the HEASARC archive and this famous image was
produced using the HEAsoft XIMAGE program.
Though individual burst time profiles are complex, and the overall
distribution of bursts is known to be isotropic but not uniform, Cline
et al. (1999, ApJ 527, 827) find that the shortest-duration bursts
have a Euclidean brightness distribution and therefore could be local.
Bonnell & Norris (2000, ApJ submitted) have shown that an apparently
homogeneous distribution for bursts with no emission detected above
300 keV is due to brightness bias in detection of bursts by BATSE.
The high time resolution of the BATSE data allows the detailed examination
of time profiles in a large sample of bursts. Scargle (1998, ApJ 504,
405) presented a promising model-independent method for the attributes of
pulses in BATSE bursts. Beloborodov et al. (2000, ApJ in press) found
that the brightnesses of bursts is correlated with the slopes of their
power density spectra, which implies that bursts are not standard candles.
Ramirez-Ruiz & Fenimore (1999, A&AS 138, 521) find the widths of pulses
in time profiles of bursts remain constant between pulses. This limits
the amount of deceleration of the expanding fireball during a burst, and
constrains any model of the emitting regions.
Spectral characteristics of bursts studied by Lloyd & Petrosian (1999,
ApJ 511, 550) suggest that BATSE may be detecting only a small
fraction of a population of hard, high-energy bursts and that the rate
of bursts at cosmological distances may be different from the star
formation rate.
Studies of afterglows of gamma-ray bursts have shown that the bursts
occur at cosmological distances. The potential for gamma-ray bursts
to be used as a probe of dark matter was recognized by Marani et
al. (1999, ApJ 512, L13), who searched BATSE data for evidence of
gravitational lensing of bursts. For massive lensing objects,
stringent upper limits on the abundance were found.
For bursts with redshifts (and hence luminosities) well-determined
from followup observations of afterglows, Norris et al. (2000, ApJ, in
press) have found a remarkable relation between spectral evolution,
specifically the energy dependence of the cross-correlation lags, and
luminosity. In addition to offering clues to the physics of bursts
this relation allows luminosity estimates for bursts that otherwise
would have unknown redshifts.
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Last modified: Monday, 19-Jun-2006 11:24:57 EDT
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