  It is now generally accepted that long-duration gamma ray bursts (GRBs) are
due to the collapse of massive rotating stars. The precise collapse process
itself, however, is not yet fully understood. Strong winds, outbursts, and
intense ionizing UV radiation from single stars or strongly interacting
binaries are expected to destroy the molecular cloud cores that give birth to
them and create highly complex circumburst environments for the explosion. Such
environments might imprint features on GRB light curves that uniquely identify
the nature of the progenitor and its collapse. We have performed numerical
simulations of realistic environments for a variety of long-duration GRB
progenitors with ZEUS-MP, and have developed an analytical method for
calculating GRB light curves in these profiles. Though a full,
three-dimensional, relativistic magnetohydrodynamical computational model is
required to precisely describe the light curve from a GRB in complex
environments, our method can provide a qualitative understanding of these
phenomena. We find that, in the context of the standard afterglow model,
massive shells around GRBs produce strong signatures in their light curves, and
that this can distinguish them from those occurring in uniform media or steady
winds. These features can constrain the mass of the shell and the properties of
the wind before and after the ejection. Moreover, the interaction of the GRB
with the circumburst shell is seen to produce features that are consistent with
observed X-ray flares that are often attributed to delayed energy injection by
the central engine. Our algorithm for computing light curves is also applicable
to GRBs in a variety of environments such as those in high-redshift
cosmological halos or protogalaxies, both of which will soon be targets of
future surveys such as JANUS or Lobster.
