The joint effect of jet and stellar wind on GRB afterglow

Gamma-ray bursts (GRBs) with long durations originate from collapse of massive stars, and this is the present popular idea about the mechanism of energy source of GRBs. Massive stars are unavoidably surrounded by stellar winds. It is also commonly believed that jets exist in GRBs. On the basis of these two viewpoints, the authors performed detailed calculations of the evolution of jets in the stellar wind environment. Because in the transition from relativistic to non-relativistic phase there are serious problems in the traditional formulae for the description of jets, some revised formulae are used. Finally, the authors' results on the evolution in the stellar wind environment are compared with those in the homogeneous medium environment, and with the results of Livio and Waxman. The following conclusions are drawn: (1) In the stellar wind environment, even for calculations up to 109s, the flux density does not show any clear tendency of levelling off. (2) In the vicinity of γ = 1/θ0 there is no break in the light curve. (Here θ0 is the initial half aperture angle.) (3) When the shock wave changes from a relativistic to a non-relativistic phase, there is a break in the light curve. (4) In comparison with the homogeneous medium, the flux density in the stellar wind environment is comparatively small.