25,342 research outputs found
Effects of Environment and Energy Injection on Gamma-Ray Burst Afterglows
There is growing evidence that some long gamma-ray bursts (GRBs) arise from
the core collapse of massive stars, and thus it is inevitable that the
environments of these GRBs are preburst stellar winds or dense media. We
studied, for the first time, the wind model for afterglows based on the
Blandford-McKee self-similar solution of a relativistic shock, and suggested
that GRB 970616 is an interactor with a stellar wind. We also proposed a dense
medium model for some afterglows, e.g., the steepening in the light curve of
the R-band afterglow of GRB 990123 may be caused by the adiabatic shock which
has evolved from an ultrarelativistic phase to a nonrelativistic phase in a
dense medium. We further discussed the dense medium model in more details, and
investigated the effects of synchrotron self absorption and energy injection. A
shock in a dense medium becomes nonrelativistic rapidly after a short
relativistic phase. The afterglow from the shock at the nonrelativistic stage
decays more rapidly than at the relativistic stage. Since some models for GRB
energy sources predicted that a strongly magnetic millisecond pulsar may be
born during GRB formation, we discussed the effect of such a pulsar on the
evolution of the nonrelativistic shock through magnetic dipole radiation. We
found that in the pulsar energy injection case, the dense medium model fits
very well all the observational data of GRB 980519. Recently, we combined the
dense medium model with the pulsar energy injection effect to provide a good
fit to the optical afterglow data of GRB 000301C.Comment: 7 pages, To appear in the proceedings of the First KIAS International
Workshop on Astrophysics: Explosive Phenomena in Astrophysical Compact
Objects, Seoul, Korea; 24-27 May 200
Behavior of X-Ray Dust Scattering and Implications for X-Ray Afterglows of Gamma-Ray Bursts
The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be
due to shocks sweeping up the circum-stellar medium. However, most GRBs have
been found in dense star-forming regions where a significant fraction of the
prompt X-ray emission can be scattered by dust grains. Here we revisit the
behavior of dust scattering of X-rays in GRBs. We find that the features of
some X-ray afterglows from minutes to days after the gamma-ray triggers are
consistent with the scattering of prompt X-ray emission from GRBs off host dust
grains. This implies that some of the observed X-ray afterglows (especially
those without sharp rising and decaying flares) could be understood with a
dust-scattering--driven emission model.Comment: ApJ, in pres
Gamma-Ray Burst Afterglows: Effects of Radiative Corrections and Nonuniformity of the Surrounding Medium
The afterglow of a gamma-ray burst (GRB) is commonly thought to be due to
continuous deceleration of a relativistically expanding fireball in the
surrounding medium. Assuming that the expansion of the fireball is adiabatic
and that the density of the medium is a power-law function of shock radius,
viz., , we analytically study the effects of the
first-order radiative correction and the nonuniformity of the medium on a GRB
afterglow. We first derive a new relation among the observed time, the shock
radius and the fireball's Lorentz factor: , and
also derive a new relation among the comoving time, the shock radius and the
fireball's Lorentz factor: . We next study the
evolution of the fireball by using the analytic solution of Blandford and McKee
(1976). The radiation losses may not significantly influence this evolution. We
further derive new scaling laws both between the X-ray flux and observed time
and between the optical flux and observed time. We use these scaling laws to
discuss the afterglows of GRB 970228 and GRB 970616, and find that if the
spectral index of the electron distribution is , implied from the
spectra of GRBs, the X-ray afterglow of GRB970616 is well fitted by assuming
.Comment: 17 pages, no figures, Latex file, MNRAS in pres
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