GRB blastwaves through wind-shaped circumburst media

Abstract

Context:A significant fraction of progenitors for long gamma-ray bursts (GRBs) are believed to be massive stars. The investigation of long GRBs therefore requires modeling the propagation of ultra-relativistic blastwaves through the circumburst medium surrounding massive stars. We simulate the expansion of an isotropic, adiabatic relativistic fireball into the wind-shaped medium around a massive GRB progenitor. The circumburst medium is composed of a realistically stratified stellar wind zone up to its termination shock, followed by a region of shocked wind characterized by a constant density. Aims: We followed the evolution of the blastwave through all its stages, including the extremely rapid acceleration up to a Lorentz factor 75 flow, its deceleration by interaction with stellar wind, its passage of the wind termination shock, until its propagation through shocked wind. Methods: We used the adaptive mesh refinement versatile advection code to follow the evolution of the fireball. Results: We show that the acceleration from purely thermal to ultra-relativistic kinetic regimes is abrupt and produces an internally structured blastwave. We resolved the structure of this ultra-relativistic shell in all stages, thanks to the adaptive mesh. We comment on the dynamical roles played by forward and reverse shock pairs in the phase of interaction with the free stellar wind and clearly identify the complex shock-dominated structure created when the shell crosses the terminal shock. Conclusion: We show that in our model where the terminal shock is taken relatively close to the massive star, the phase of self-similar deceleration of Blandford-McKee type can only be produced in the constant density, shocked wind zone.