The circumburst density profile around GRB progenitors: a statistical
  study
  According to our present understanding, long GRBs originate from the collapse
of massive stars while short bursts are due to the coalescence of compact
stellar objects. Since the afterglow evolution is determined by the circumburst
density profile, n(r), traversed by the fireball, it can be used to distinguish
between a so-called ISM profile, n(r) = const., and a free stellar wind, $n(r)
\propto r^{-2}$. Our goal is to derive the most probable circumburst density
profile for a large number of Swift-detected bursts using well-sampled
afterglow light curves in the optical and X-ray bands. We combined all publicly
available optical and Swift/X-ray afterglow data from June 2005 to September
2009 to find the best-sampled late-time afterglow light curves. After applying
several selection criteria, our final sample consists of 27 bursts, including
one short burst. The afterglow evolution was then studied within the framework
of the fireball model. We find that the majority (18) of the 27 afterglow light
curves are compatible with a constant density medium (ISM case). Only 6 of the
27 afterglows show evidence for a wind profile at late times. In particular, we
set upper limits on the wind termination-shock radius, $R_T$, for GRB fireballs
which are propagating into an ISM profile and lower limits on $R_T$ for those
which were found to propagate through a wind medium. Observational evidence for
ISM profiles dominates in GRB afterglow studies, implying that most GRB
progenitors might have relatively small wind termination-shock radii. A smaller
group of progenitors, however, seems to be characterised by notably more
extended wind regions.
