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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
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