17,877 research outputs found
Can the jet steepen the light curves of GRB afterglow?
Beaming of relativistic ejecta in GRBs has been postulated by many authors in
order to reduce the total GRB energy, thus it is very important to look for the
observational evidence of beaming. Rhoads (1999) has pointed out that the
dynamics of the blast wave, which is formed when the beamed ejecta sweeping the
external medium, will be significantly modified by the sideways expansion due
to the increased swept up matter. He claimed that shortly after the bulk
Lorentz factor () of the blast wave drops below the inverse of the
initial opening angle () of the beamed ejecta, there will be a
sharp break in the afterglow light curves. However, some other authors have
performed numerical calculations and shown that the break of the light curve is
weaker and much smoother than the one analytically predicted. In this paper we
reanalyse the dynamical evolution of the jet blast wave, calculate the jet
emission analytically, we find that the sharp break predicted by Rhoads will
actually not exist, and for most cases the afterglow light curve will almost
not be affected by sideways expansion unless the beaming angle is extremely
small. We demonstrate that only when , the afterglow light
curves may be steepened by sideways expansion, and in fact there cannot be two
breaks as claimed before. We have also constructed a simple numerical code to
verify our conclusion.Comment: 12 pages, 2 figures, accepted by ApJ, added numerical calculation
Diverse Temporal Properties of GRB Afterglow
The detection of delayed X-ray, optical and radio emission, "afterglow",
associated with -ray bursts (GRBs) is consistent with fireball models,
where the emission are produced by relativistic expanding blast wave, driven by
expanding fireball at cosmogical distances. The emission mechanisms of GRB
afterglow have been discussed by many authors and synchrotron radiation is
believed to be the main mechanism. The observations show that the optical light
curves of two observed gamma-ray bursts, GRB970228 and GRB GRB970508, can be
described by a simple power law, which seems to support the synchrotron
radiation explanation. However, here we shall show that under some
circumstances, the inverse Compton scattering (ICS) may play an important role
in emission spectrum and this may influence the temporal properties of GRB
afterglow. We expect that the light curves of GRB afterglow may consist of
multi-components, which depends on the fireball parameters.Comment: Latex, no figures, minor correctio
Gamma-ray bursts: postburst evolution of fireballs
The postburst evolution of fireballs that produce -ray bursts is
studied, assuming the expansion of fireballs to be adiabatic and relativistic.
Numerical results as well as an approximate analytic solution for the evolution
are presented. Due to adoption of a new relation among , and
(see the text), our results differ markedly from the previous studies.
Synchrotron radiation from the shocked interstellar medium is attentively
calculated, using a convenient set of equations. The observed X-ray flux of GRB
afterglows can be reproduced easily. Although the optical afterglows seem much
more complicated, our results can still present a rather satisfactory approach
to observations. It is also found that the expansion will no longer be highly
relativistic about 4 days after the main GRB. We thus suggest that the
marginally relativistic phase of the expansion should be investigated so as to
check the afterglows observed a week or more later.Comment: 17 pages, 4 figures, MNRAS in pres
Is GRO J1744-28 a Strange Star?
The unusal hard x-ray burster GRO J1744-28 recently discovered by the Compton
Gamma-ray Observatory (GRO) can be modeled as a strange star with a dipolar
magnetic field Gauss. When the accreted mass of the star exceeds
some critical mass, its crust may break, resulting in conversion of the
accreted matter into strange matter and release of energy. Subsequently, a
fireball may form and expand relativistically outward. The expanding fireball
may interact with the surrounding interstellar medium, causing its kinetic
energy to be radiated in shock waves, producing a burst of x-ray radiation. The
burst energy, duration, interval and spectrum derived from such a model are
consistent with the observations of GRO J1744-28.Comment: Latex, has been published in SCIENCE, Vol. 280, 40
Nuclear Magnetic Resonance Implementation of a Quantum Clock Synchronization Algorithm
The quantum clock synchronization algorithm proposed by I. L. Chuang (Phys.
Rev. Lett, 85, 2006(2000)) has been implemented in a three qubit nuclear
magnetic resonance quantum system. The effective-pure state is prepared by the
spatial averaging approach. The time difference between two separated clocks
can be determined by reading out directly through the NMR spectra.Comment: 13 pages, 5 figure
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