  We present hydrodynamical models of circumstellar medium (CSM) of long
gamma-ray burst (GRB) progenitor candidates. These are massive stars that have
lost a large amount of mass in the form of stellar wind during their evolution.
  There are two possible ways to probe the CSM of long GRB progenitors.
Firstly, the GRB afterglow consists of synchrotron radiation, emitted when the
GRB jet sweeps up the surrounding medium. Therefore, the lightcurve is directly
related to the density profile of the CSM. The density can either decrease with
the radius squared (as is the case for a freely expanding stellar wind) or be
constant (as we would expect for shocked wind or the interstellar medium).
Secondly, material between the GRB and the observer will absorb part of the
afterglow radiation, causing absorption lines in the afterglow spectrum. In
some cases, such absorption lines are blue-shifted relative to the source
indicating that the material is moving away from the progenitor star. This can
be explained in terms of wind interactions in the CSM. We can use the CSM of
these stars to investigate their prior evolutionary stage.
