2006MNRAS.372.1791L

A high-resolution spectroscopy of GRB 021004 revealed a wealth of absorption lines from several intermediate-ionization species. The velocity structure of the absorber is complex and material with velocity up to ≳3000km/s is observed. Since only the blueshifted component is observed, the absorber is very likely to be material closely surrounding the gamma-ray burst. We use a time-dependent photoionization code to track the abundance of the ions over time. Thanks to the presence of absorption from intermediate-ionization states at long times, we can estimate the location and mass of the components of the absorber. We interpret those constraints within the hypernova scenario showing that the mass-loss rate of the progenitor must have been ∼10^–4M_☉/yr, suggestive of a very massive star. In addition, the wind termination shock must lie at a distance of at least 100 pc, implying a low-density environment. The velocity structure of the absorber also requires clumping of the wind at those large distances.