Winds from massive stars: Implications for the afterglows of γ-ray bursts

Recent observations suggest that long-duration γ-ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. The pre-explosive ambient medium provides a natural test for the most likely progenitors of GRBs. Those stars that shed their envelopes most readily have short jet crossing times and are more likely to produce a GRB. We construct a simple computational scheme to explore the expected contribution of the presupernova ejecta of single Wolf-Rayet (WR) stars to the circumstellar environment. Using detailed stellar tracks for the evolution of massive stars, we discuss the effects that the initial main-sequence mass, metallicity, rotation and membership in a binary system have on the ambient medium. We extend the theory of GRB afterglows in winds to consider the effect of the relativistic fireball propagating through the WR ejecta. Specific predictions are made for the interaction of the relativistic blast wave with the density bumps that arise when the progenitor star rapidly loses a large fraction of its initial mass or when the ejected wind interacts with the external medium and decelerates. A re-brightening of the afterglow with a spectrum redder than the typical synchrotron spectrum (as seen in GRB 970508, GRB 980326 and GRB 000911) is predicted. We also calculate the luminosity of the reflected echo that arises when circumstellar material Compton-scatters the prompt radiation, and examine the spectral signatures expected from the interaction of the GRB afterglow with the ejected medium.