1,814 research outputs found
An Observational Evidence for the Difference Between the Short and Long Gamma-Ray Bursts
The intrinsic fluence and duration distributions of gamma-ray bursts are well represented by log-normal
distributions. This allows a bivariate log-normal distribution fit to be made to the BATSE short and long bursts
separately. A statistically significant difference between the long and short groups is found. We argue that the
effect is probably real. Applying the CramĂŠrâs theorem these results lead to some predictions for models of long
and short bursts
'On the difference between the short and long gamma-ray bursts'
We argue that the distributions of both the intrinsic fluence and the
intrinsic duration of the gamma-ray emission in gamma-ray bursts from the BATSE
sample are well represented by log-normal distributions, in which the intrinsic
dispersion is much larger than the cosmological time dilatation and redshift
effects. We perform separate bivariate log-normal distribution fits to the
BATSE short and long burst samples. The bivariate log-normal behaviour results
in an ellipsoidal distribution, whose major axis determines an overall
statistical relation between the fluence and the duration. We show that this
fit provides evidence for a power-law dependence between the fluence and the
duration, with a statistically significant different index for the long and
short groups. We discuss possible biases, which might affect this result, and
argue that the effect is probably real. This may provide a potentially useful
constraint for models of long and short bursts.Comment: A.A. in press ; significantly revised version of astro-ph/0007438; 16
pages 5 PS figure
Gamma photometric redshifts for long gamma-ray bursts
It is known that the soft tail of the gamma-ray bursts' spectra show excesses
from the exact power-law dependence. In this article we show that this
departure can be detected in the peak flux ratios of different BATSE DISCSC
energy channels. This effect allows to estimate the redshift of the bright long
gamma-ray bursts in the BATSE Catalog. A verification of these redshifts is
obtained for the 8 GRB which have both BATSE DISCSC data and measured optical
spectroscopic redshifts. There is good correlation between the measured and
esti redshifts, and the average error is . The method is
similar to the photometric redshift estimation of galaxies in the optical
range, hence it can be called as "gamma photometric redshift estimation". The
estimated redshifts for the long bright gamma-ray bursts are up to . For the the faint long bursts - which should be up to - the
redshifts cannot be determined unambiguously with this method.Comment: accepted in A&A, 7 pages incl. 7 figure
Anisotropy of the sky distribution of gamma-ray bursts
The isotropy of gamma-ray bursts collected in current BATSE catalog is studied. It is shown that the quadrupole term being proportional to \sim sin 2b sin l is non-zero with a probability of 99.9%. The occurrence of this anisotropy term is then confirmed by the binomial test even with the probability of 99.97 %. Hence, the sky distribution of all known gamma-ray bursts is anisotropic. It is also argued that this anisotropy cannot be caused exclusively by instrumental effects due to the nonuniform sky exposure of BATSE instrument. Separating the GRBs into short and long subclasses, it is shown that the short ones are distributed anisotropically, but the long ones seem to be distributed still isotropically. The character of anisotropy suggests that the cosmological origin of short GRBs further holds, and there is no evidence for their Galactical origin
Power Density Spectra of Gamma-Ray Bursts in the Internal Shock Model
We simulate Gamma-Ray Bursts arising from internal shocks in relativistic
winds, calculate their power density spectrum (PDS), and identify the factors
to which the PDS is most sensitive: the wind ejection features, which determine
the wind dynamics and its optical thickness, and the energy release parameters,
which give the pulse 50-300 keV radiative efficiency. For certain combinations
of ejection features and wind parameters the resulting PDS exhibits the
features observed in real bursts. We found that the upper limit on the
efficiency of conversion of wind kinetic energy into 50-300 keV photons is
1%. Winds with a modulated Lorentz factor distribution of the ejecta
yield PDSs in accord with current observations and have efficiencies closer to
, while winds with a random, uniform Lorentz factor ejection must be
optically thick to the short duration pulses to produce correct PDSs, and have
an overall efficiency around .Comment: 6 pages, 4 figures, Latex, submitted to The Astrophysical Journal
(05/04/99
Gamma-Ray Bursts from Up-Scattered Self-Absorbed Synchrotron Emission
We calculate the synchrotron self-Compton emission from internal shocks
occurring in relativistic winds as a source of gamma-ray bursts, with allowance
for self-absorption. For plausible model parameters most pulses within a
Gamma-Ray Burst (GRB) are optically thick to synchrotron self-absorption at the
frequency at which most electrons radiate. Up-scattering of photon number
spectra harder than (such as the self-absorbed emission) yields inverse
Compton photon number spectra that are flat, therefore our model has the
potential of explaining the low-energy indices harder than (the
optically thin synchrotron limit) that have been observed in some bursts. The
optical counterparts of the model bursts are sufficiently bright to be detected
by such experiments as LOTIS, unless the magnetic field is well below
equipartition.Comment: to be published in ApJL, 5 pages, 3 color figure
An intrinsic anisotropy in the angular distribution of gamma-ray bursts
The anisotropy of the sky distribution of 2025 gamma-ray bursts (GRBs) collected in Current BATSE catalog is confirmed. It is shown that the quadrupole term being proportional to similar to sin 2b sin I is non-zero with a probability 99.9%. The occurrence of this anisotropy term is then supported by the binomial test even with the probability 99.97%. It is also argued that this anisotropy cannot be caused exclusively by instrumental effects due to the non-uniform sky exposure of BATSE instrument; there should exist also some intrinsic anisotropy in the angular distribution of GRBs. Separating GRBs into short and long subclasses, it is shown that the 251 short ones are distributed anisotropically, but the 681 long ones seem to be distributed still isotropically. The 2-sample Kolmogorov Smirnov test shows that they are distributed differently with a 98.7% probability. The character of anisotropy suggests that the cosmological origin of short GRBs further holds, and there is no evidence for their Galactical origin. The work in essence contains the key ideas and results of a recently published paper (Balazs et al. 1998), to which the new result following from the 2-sample Kolmogorov-Smirnov test is added, too
Testing the randomness in the sky-distribution of gamma-ray bursts
We have studied the complete randomness of the angular distribution of gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE). Because GRBs seem to be a mixture of objects of different physical nature, we divided the BATSE sample into five subsamples (short1, short2, intermediate, long1, long2) based on their durations and peak fluxes, and we studied the angular distributions separately. We used three methods, Voronoi tesselation, minimal spanning tree and multifractal spectra, to search for non-randomness in the subsamples. To investigate the eventual non-randomness in the subsamples, we defined 13 test variables (nine from the Voronoi tesselation, three from the minimal spanning tree and one from the multifractal spectrum). Assuming that the point patterns obtained from the BATSE subsamples are fully random, we made Monte Carlo simulations taking into account the BATSE's sky-exposure function. The Monte Carlo simulations enabled us to test the null hypothesis (i.e. that the angular distributions are fully random). We tested the randomness using a binomial test and by introducing squared Euclidean distances in the parameter space of the test variables. We concluded that the short1 and short2 groups deviate significantly (99.90 and 99.98 per cent, respectively) from the full randomness in the distribution of the squared Euclidean distances; however, this is not the case for the long samples. For the intermediate group, the squared Euclidean distances also give a significant deviation (98.51 per cent)
Extended GeV-TeV Emission around Gamma-Ray Burst Remnants, and the Case of W49B
We investigate the high-energy photon emission around Gamma-Ray Burst (GRB)
remnants caused by ultrahigh-energy cosmic rays (UHECRs) from the GRBs. We
apply the results to the recent report that the supernova remnant W49B is a GRB
remnant in our Galaxy. If this is correct, and if GRBs are sources of UHECRs, a
natural consequence of this identification would be a detectable TeV photon
emission around the GRB remnant. The imaging of the surrounding emission could
provide new constraints on the jet structure of the GRB.Comment: 12 pages, 2 figures, accepted for publication in ApJ
The Rees-Sciama effect and the primordial nucleosynthesis
It is known that, theoretically, the Rees-Sciama effect may cause arbitrarily
large additional redshifts in the cosmic microwave background radiation due to
transparent expanding voids having sizes comparable with the size of horizon.
Therefore, again theoretically, eventual huge voids existing immediately after
the recombination may essentially change the predictions of the theory of big
bang nucleosynthesis. If this eventuality holds, then the dark matter can be
dominantly baryonic and, simultaneously, one can be in accordance with the
predictions of primordial nucleosynthesis theory. Studying this eventuality one
arrives at the result that the observed extreme isotropy of the cosmic
microwave background radiation rejects the existence of any such huge voids,
and hence this eventuality does not hold.Comment: Astronomy and Astrophysics, accepted for publication, 4 page
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