423 research outputs found
The Temporal and Spectral Characteristics of "Fast Rise and Exponential Decay" Gamma-Ray Burst Pulses
In this paper we have analyzed the temporal and spectral behavior of 52 Fast
Rise and Exponential Decay (FRED) pulses in 48 long-duration gamma-ray bursts
(GRBs) observed by the CGRO/BATSE, using a pulse model with two shape
parameters and the Band model with three shape parameters, respectively. It is
found that these FRED pulses are distinguished both temporally and spectrally
from those in long-lag pulses. Different from these long-lag pulses only one
parameter pair indicates an evident correlation among the five parameters,
which suggests that at least 4 parameters are needed to model burst
temporal and spectral behavior. In addition, our studies reveal that these FRED
pulses have correlated properties: (i) long-duration pulses have harder spectra
and are less luminous than short-duration pulses; (ii) the more asymmetric the
pulses are the steeper the evolutionary curves of the peak energy () in
the spectrum within pulse decay phase are. Our statistical
results give some constrains on the current GRB models.Comment: 18 pages, 7 figures, accepted for publication in the Astrophysical
Journa
On the Origin of the Dark Gamma-Ray Bursts
The origin of dark bursts - i.e. that have no observed afterglows in X-ray,
optical/NIR and radio ranges - is unclear yet. Different possibilities -
instrumental biases, very high redshifts, extinction in the host galaxies - are
discussed and shown to be important. On the other hand, the dark bursts should
not form a new subgroup of long gamma-ray bursts themselves.Comment: published in Nuovo Ciment
Factor analysis of the long gamma-ray bursts
We study statistically 197 long gamma-ray bursts, detected and measured in
detail by the BATSE instrument of the Compton Gamma-Ray Observatory. In the
sample 10 variables, describing for any burst the time behavior of the spectra
and other quantities, are collected. The factor analysis method is used to find
the latent random variables describing the temporal and spectral properties of
GRBs. The application of this particular method to this sample indicates that
five factors and the \REpk spectral variable (the ratio of peak energies in
the spectrum) describe the sample satisfactorily. Both the pseudo-redshifts
inferred from the variability, and the Amati-relation in its original form, are
disfavored.Comment: 5 pages, acceptod to A&
Autocorrelation analysis of GRBMâBeppo-SAX burst data
An autocorrelation function (ACF) analysis was performed on 17 gamma-ray bursts with known redshift, using data from the GRBM on board Beppo-SAX. When corrected from the cosmic time dilation effect, the ACFs show a bimodal distribution at about half-maximum, in agreement with a previous study based on BATSE and Konus burst data. Although the results show more dispersion, the separation between the two classes is highly significant
Time-resolved spectral correlations of long-duration Gamma-Ray Bursts
For a sample of long GRBs with known redshift, we study the distribution of
the evolutionary tracks on the rest-frame luminosity-peak energy Liso-Ep'
diagram. We are interested in exploring the extension of the `Yonetoku'
correlation to any phase of the prompt light curve, and in verifying how the
high-signal prompt duration time, Tf, in the rest frame correlates with the
residuals of such correlation (Firmani et al. 2006). For our purpose, we
analyse separately two samples of time-resolved spectra corresponding to 32
GRBs with peak fluxes >1.8 phot cm^-2 s^-1 from the Swift-BAT detector, and 7
bright GRBs from the CGRO-BATSE detector previously processed by Kaneko et al.
(2006). After constructing the Liso-Ep' diagram, we discuss the relevance of
selection effects, finding that they could affect significantly the
correlation. However, we find that these effects are much less significant in
the Liso x Tf-Ep' diagram, where the intrinsic scatter reduces significantly.
We apply further corrections for reducing the intrinsic scatter even more. For
the sub-samples of GRBs (7 from Swift and 5 from CGRO) with measured jet break
time, we analyse the effects of correcting Liso by jet collimation. We find
that (i) the scatter around the correlation is reduced, and (ii) this scatter
is dominated by the internal scatter of the individual evolutionary tracks.
These results suggest that the time, integrated `Amati' and `Ghirlanda'
correlations are consequences of the time resolved features, not of selection
effects, and therefore call for a physical origin. We finally remark the
relevance of looking inside the nature of the evolutionary tracks.Comment: 11 pages, 6 figures, 4 tables. Submitted to MNRAS (Sept 8th), after
referee comment
Principal-Component Analysis of Gamma-Ray Burstsâ Spectra
The principal-component analysis is a statistical method, which lowers the number of important variables in a data set. The use of this method for the burstsâ spectra and afterglows is discussed in this paper. The analysis indicates that three principal components are enough among the eight ones to describe the variablity of the data. The correlation between the spectral index α and the redshift
suggests that the thermal emission component becomes more dominant at larger redshifts
On the temporal variability classes found in long gamma-ray bursts with known redshift
Based on the analysis of a small sample of BATSE and Konus gamma-ray bursts
(GRBs) with know redshift it has been reported that the width of the
autocorrelation function (ACF) shows a remarkable bimodal distribution in the
rest-frame of the source. However, the origin of these two well-separated ACF
classes remains unexplained.We complement previous ACF analysis studying the
corresponding power density spectra (PDS). With the addition of Beppo-SAX data
and taken advantage of its broad-band capability, we not only increase the
burst sample but we extend the analysis to X-ray energies. The rest-frame PDS
analysis at gamma-ray energies shows that the two ACF classes are not simply
characterised by a different low frequency cut-off, but they have a distinct
variability as a whole in the studied frequency range. Both classes exhibit
average PDS with power-law behaviour at high frequencies (f' > 0.1 Hz) but
significantly different slopes, with index values close to those of Brownian
(-2) and Kolmogorov (-5/3) spectra for the narrow and broad classes
respectively. The latter spectrum presents an additional PDS component, a
low-frequency noise excess with a sharp cut-off. At X-ray energies we find the
power-law index unchanged for the broad class, but a significantly steeper
slope in the narrow case (~ -3). We interpret this as an indication that the
broad class bursts have weaker spectral evolution than the narrow ones, as
suggested also by our analysis of the ACF energy dependence. The low and high
frequency PDS components may then arise from two radiating regions involving
different emission mechanisms.Comment: 13 pages, 10 figures. Accepted for publication in A&
Bimodal distribution of the autocorrelation function in gamma-ray bursts
Autocorrelation functions (ACFs) are studied for a sample of 16 long
gamma-ray bursts (GRBs) with known redshift z, that were observed by the BATSE
and Konus experiments. When corrected for cosmic time dilation, the ACF shows a
bimodal distribution. A narrow width class (11 bursts) has at half-maximum a
mean width of 1.6 s with a relative dispersion of 32%, while a broad width
class (5 bursts) has a width of 7.5 s with a 4% dispersion. The separation
between the two mean values is highly significant (> 7 standard deviations).
This temporal property could be used on the large existing database of GRBs
with unknown redshift. The broad width set shows a very good linear correlation
between width at half-maximum and (1+z), with a correlation coefficient R=0.995
and a probability of chance alignment <0.0004. The potential application of
this correlation to cosmology studies is discussed, using it in combination
with recently proposed luminosity indicators.Comment: 7 pages, 7 figures. Accepted for publication in A&
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