  The association of long gamma-ray bursts (GRBs) with Type Ib/c supernovae
implies that they explode into the winds of their Wolf-Rayet progenitor stars.
Although the evolution of some GRB afterglows is consistent with expansion into
a free wind, there is also good evidence for expansion into a constant density
medium. The evidence includes the evolution of X-ray afterglows (when X-rays
are below the cooling frequency), the evolution of the pre-jet break optical
and X-ray afterglow, and the sharp turn-on observed for some afterglows. Recent
observations of short bursts, which are expected to be interacting with a
constant density medium, provide a check on the standard afterglow model.
Although radio observations do not support the constant density model for long
bursts in some cases, the evidence for constant density interaction is strong.
The most plausible way to produce such a medium around a massive star is to
shock the progenitor wind. This requires a smaller termination shock than would
be expected, possibly due to a high pressure surroundings, a high progenitor
velocity, or the particular evolution leading to a GRB. However, the need for
the termination shock near the deceleration radius cannot be plausibly
accomodated and may indicate that some long bursts have compact binary
progenitors and explode directly into the interstellar medium.
