The circumburst density profile around GRB progenitors

Observational and theoretical evidence suggest that long gamma-ray bursts (GRBs) originate from the collapse of massive stars, while short bursts are caused by the coalescence of stellar compact objects. Finding direct evidence for both scenarios is challenging. Intriguingly, the afterglow evolution depends on the density profile of the circumburst medium (CBM). We use this property to find the most probable CBM for a large number of Swift-detected bursts with well-sampled optical and X-ray light curves. In our study we distinguish between a constant density medium, n(r) = const., and a free stellar wind, n(r) ∞ r^-2. After applying several selection criteria, our sample consists of 27 bursts, including one short burst. The majority (18) of the 27 afterglows favor an ISM profile; in only six bursts a wind medium at late times is preferred. Furthermore, we set limits on the wind-termination shock radius. Bursts favoring a constant density environment also favor smaller wind-termination shock radii than bursts traversing a free stellar wind at late times.