27,994 research outputs found
Gamma-Ray Burst Afterglows from Realistic Fireballs
A GRB afterglow has been commonly thought to be due to continuous
deceleration of a postburst fireball. Many analytical models have made
simplifications for deceleration dynamics of the fireball and its radiation
property, although they are successful at explaining the overall features of
the observed afterglows. We here propose a model for a GRB afterglow in which
the evolution of a postburst fireball is in an intermediate case between the
adiabatic and highly radiative expansion. In our model, the afterglow is both
due to the contribution of the adiabatic electrons behind the external
blastwave of the fireball and due to the contribution of the radiative
electrons. In addition, this model can describe evolution of the fireball from
the extremely relativistic phase to the non-relativistic phase. Our
calculations show that the fireball will go to the adiabatic expansion phase
after about a day if the accelerated electrons are assumed to occupy the total
internal energy. In all cases considered, the fireball will go to the mildly
relativistic phase about seconds later, and to the non-relativistic
phase after several days. These results imply that the relativistic adiabatic
model cannot describe the deceleration dynamics of the several-days-later
fireball. The comparison of the calculated light curves with the observed
results at late times may imply the presence of impulsive events or energy
injection with much longer durations.Comment: 18 pages, 10 figures, plain latex file, submitted to Ap
Measuring dark energy with the correlation of gamma-ray bursts using model-independent methods
In this paper, we use two model-independent methods to standardize long
gamma-ray bursts (GRBs) using the correlation, where
is the isotropic-equivalent gamma-ray energy and is
the spectral peak energy. We update 42 long GRBs and try to make constraint on
cosmological parameters. The full sample contains 151 long GRBs with redshifts
from 0.0331 to 8.2. The first method is the simultaneous fitting method. The
extrinsic scatter is taken into account and assigned to the
parameter . The best-fitting values are ,
, and in the flat
CDM model. The constraint on is at the
1 confidence level. If reduced method is used, the best-fit
results are , and . The
second method is using type Ia supernovae (SNe Ia) to calibrate the correlation. We calibrate 90 high-redshift GRBs in the redshift
range from 1.44 to 8.1. The cosmological constraints from these 90 GRBs are
for flat CDM, and
and for non-flat
CDM. For the combination of GRB and SNe Ia sample, we obtain
and for the flat CDM, and
for the non-flat CDM, the results are ,
and . These results from
calibrated GRBs are consistent with that of SNe Ia. Meanwhile, the combined
data can improve cosmological constraints significantly, comparing to SNe Ia
alone. Our results show that the correlation is
promising to probe the high-redshift universe.Comment: 10 pages, 6 figures, 4 table, accepted by A&A. Table 4 contains
calibrated distance moduli of GRB
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