2000ApJ...538..187L

We consider models for gamma-ray bursts in which a collimated jet expands either into a homogeneous medium or into a stellar wind environment, and we calculate the expected afterglow temporal behavior. We show that (1) following a break and a faster decay afterglows should exhibit a flattening, which may be detectable in both the radio and optical bands, and (2) only observations at times much shorter than a day can clearly distinguish between a fireball interacting with a homogeneous medium and one interacting with a stellar wind. Using our results, we demonstrate that constraints can be placed on progenitor models. In particular, existing data imply that while some long-duration bursts may be produced by collapses of massive stars, it is almost certain that not all long-duration bursts are produced by such progenitors.