1,505 research outputs found
Peak Energy-Isotropic Energy Relation in the Off-Axis Gamma-Ray Burst Model
Using a simple uniform jet model of prompt emissions of gamma-ray bursts
(GRBs), we reproduce the observed peak energy--isotropic energy relation. A
Monte Carlo simulation shows that the low-isotropic energy part of the relation
is dominated by events viewed from off-axis directions, and the number of the
off-axis events is about one-third of the on-axis emissions. We also compute
the observed event rates of the GRBs, the X-ray-rich GRBs, and the X-ray
flashes detected by HETE-2, and we find that they are similar.Comment: 11 pages, 2 eps figure
Delayed Flashes from Counter Jets of Gamma Ray Bursts
If X-ray flashes are due to the forward jet emissions from gamma ray bursts
(GRBs) observed with large viewing angles, we show that a prompt emission from
a counter jet should be observed as a delayed flash in the UV or optical band
several hours to a day after the X-ray flash. Ultraviolet and Optical Telescope
on Swift can observe the delayed flashes within ~13 Mpc, so that (double-sided)
jets of GRBs may be directly confirmed. Since the event rate of delayed flashes
detected by Swift may be as small as 6*10^{-5}events/yr, we require more
sensitive detectors in future experiments.Comment: 15 pages, 4 eps figures, Accepted for publication in Ap
Cosmological X-Ray Flashes from Off-Axis Jets
The of the cosmological X-ray flashes detected by WFC/BeppoSAX is
calculated theoretically in a simple jet model. The total emission energy from
the jet is assumed to be constant. We find that if the jet opening half-angle
is smaller than 0.03 radian, off-axis emission from sources at z<~4 can be
seen. The theoretical is less than 0.4, which is consistent with the
observational result of 0.27+/-0.16 at the 1-sigma level. This suggests that
the off-axis GRB jet with the small opening half-angle at the cosmological
distance can be identified as the cosmological X-ray flash.Comment: 4 pages, 3 figures aipTEX, contribution to the 2003 GRB Conference,
held at Santa Fe, N
GRB980425 in the Off-Axis Jet Model of the Standard GRBs
Using a simple off-axis jet model of GRBs, we can reproduce the observed
unusual properties of the prompt emission of GRB980425, such as the extremely
low isotropic equivalent gamma-ray energy, the low peak energy, the high
fluence ratio, and the long spectral lag when the jet with the standard energy
of ~10^{51} ergs and the opening half-angle of \Delta\theta=~10-30 degree is
seen from the off-axis viewing angle ~\Delta\theta+10/\gamma, where \gamma is a
Lorentz factor of the jet. For our adopted fiducial parameters, if the jet that
caused GRB 980425 is viewed from the on-axis direction, the intrinsic peak
energy Ep(1+z) is ~2.0-4.0 MeV, which corresponds to those of GRB990123 and
GRB021004. Our model might be able to explain the other unusual properties of
this event. We also discuss the connection of GRB980425 in our model with the
X-ray flash, and the origin of a class of GRBs with small E_\gamma such as
GRB030329, GRB980329, GRB981226, and so on.Comment: 4 pages, 2 figures aipTEX, contribution to the 2003 GRB Conference,
held at Santa Fe, N
A possible observational evidence for angular distribution of opening half-angle of GRB jets
We propose a method to estimate the pseudo jet opening half-angle of GRBs
using the spectral peak energy (\Ep)--peak luminosity relation (so called
Yonetoku relation) as well as the \Ep--collimation-corrected -ray
energy relation (so called Ghirlanda relation). For bursts with known jet break
times and redshifts, we compared the pseudo jet opening half-angle with the
standard one and found that the differences are within a factor 2. We apply the
method to 689 long GRBS. We found that the distribution function of the pseudo
jet opening half-angle obeys with
possible cutoffs for although the
log-normal fit is also possible. distribution is compatible with
the structured jet model. From the distribution function we found that the
beaming correction for the rate of GRBs is , which means yr galaxy or only one in type Ib/c supernovae. We
also found the evolution of the distribution function as a function of the
redshift.Comment: 5 pages, 5 figures, submitted to MNRA
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