GRB progenitors and their environments
  The detection of supernova features in the late spectra of several gamma-ray burst afterglows has shown that at least a fraction of long-duration gamma-ray bursts are associated to the final evolutionary stages of massive stars. Such direct observations are impossible for bursts located at redshift beyond z~0.5 and different methods must be used to understand the nature and properties of their progenitors. We review the observational evidence for two particular bursts, for which high quality data are available: GRB 021004 (at z=2.323) and the record redshift GRB 050904 (at z=6.29). We show that both GRBs are likely to be associated to very massive stars, and that in both cases the progenitor stars were able to modify their immediate environments with their radiative and mechanic (wind) luminosity. 
Davide Lazzati (JILA, University of Colorado)