Models of the circumstellar medium around gamma-ray burst progenitors

We present two ways to obtain information on the circumstellar medium of long γ-Ray Burst (GRB) progenitors. One: if the afterglow spectrum of the GRB shows blue-shifted absorption lines, these can be related to hydrodynamical interactions between the stellar wind of the progenitor and its environment. Two: the afterglow light curve is directly related to the density profile of the circumburst medium. This can either decrease with the square of the distance as should be expected for an expanding stellar wind medium or be constant, in which case we think that the GRB jet is expanding in a shocked wind region. In turn, an understanding of the circumstellar medium of the GRB progenitor can help understand the evolution of the progenitor star.