1,106 research outputs found
A Principal Component Analysis of the 3B Gamma-Ray Burst Data
We have carried out a principal component analysis for 625 gamma-ray bursts
in the BATSE 3B catalog for which non-zero values exist for the nine measured
variables. This shows that only two out of the three basic quantities of
duration, peak flux and fluence are independent, even if this relation is
strongly affected by instrumental effects, and these two account for 91.6% of
the total information content. The next most important variable is the fluence
in the fourth energy channel (at energies above 320 keV). This has a larger
variance and is less correlated with the fluences in the remaining three
channels than the latter correlate among themselves. Thus a separate
consideration of the fourth channel, and increased attention on the related
hardness ratio appears useful for future studies. The analysis gives the
weights for the individual measurements needed to define a single duration,
peak flux and fluence. It also shows that, in logarithmic variables, the
hardness ratio is significantly correlated with peak flux, while is
significantly anticorrelated with peak flux. The principal component analysis
provides a potentially useful tool for estimating the improvement in
information content to be achieved by considering alternative variables or
performing various corrections on available measurementsComment: Ap.J., accepted 12/9/97; revised version contains a new appendix,
somewhat expanded discussion; latex, aaspp4, 15 page
GRB Precursors in the Fallback Collapsar Scenario
Precursor emission has been observed in a non-negligible fraction of
gamma-ray bursts.The time gap between the precursor and the main burst extends
in some case up to hundreds of seconds, such as in GRB041219A, GRB050820A and
GRB060124. Both the origin of the precursor and the large value of the time gap
are controversial. Here we investigate the maximum possible time gaps arising
from the jet propagation inside the progenitor star, in models which assume
that the precursor is produced by the jet bow shock or the cocoon breaking out
of the progenitor. Due to the pressure drop ahead of the jet head after it
reaches the stellar surface, a rarefaction wave propagates back into the jet at
the sound speed, which re-accelerates the jet to a relativistic velocity and
therefore limits the gap period to within about ten seconds. This scenario
therefore cannot explain gaps which are hundreds of seconds long. Instead, we
ascribe such long time gaps to the behavior of the central engine, and suggest
a fallback collapsar scenario for these bursts. In this scenario, the precursor
is produced by a weak jet formed during the initial core collapse, possibly
related to MHD processes associated with a short-lived proto-neutron star,
while the main burst is produced by a stronger jet fed by fallback accretion
onto the black hole resulting from the collapse of the neutron star. We have
examined the propagation times of the weak precursor jet through the stellar
progenitor. We find that the initial weak jet can break out of the progenitor
in a time less than ten seconds (a typical precursor duration) provided that it
has a moderately high relativistic Lorentz factor \Gamma>=10 (abridged).Comment: 8 pages, accepted by ApJ, this version contains significantly
expanded discussion and an additional figure, conclusions unchange
Properties of the intermediate type of gamma-ray bursts
Gamma-ray bursts can be divided into three groups ("short", "intermediate",
"long") with respect to their durations. The third type of gamma-ray bursts -
as known - has the intermediate duration. We show that the intermediate group
is the softest one. An anticorrelation between the hardness and the duration is
found for this subclass in contrast to the short and long groups.Comment: In Sixteenth Maryland Astrophysics Conferenc
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