Spectroscopy of GRB 021004: a structured jet ploughing through a massive stellar wind

Abstract

We present spectra of the afterglow of GRB 021004 taken with WHT ISIS and VLT FORS1 at three epochs spanning 0.49--6.62 days after the burst. We observe strong absorption likely coming from the host galaxy, alongside absorption in HI, SiIV and CIV with blueshifts of up to 2900 km/s from the explosion centre which we assume originates close to the progenitor. We find no significant variability of these spectral features. We investigate the origin of the outflowing material and evaluate various possible progenitor models. The most plausible explanation is that these result in the fossil stellar wind of a highly evolved Wolf-Rayet star. However, ionization from the burst itself prevents the existence of HI, SiIV and CIV close to the afterglow surface where the fast stellar wind should dominate, and large amounts of blueshifted hydrogen are not expected in a Wolf-Rayet star wind. We propose that the Wolf-Rayet star wind is enriched by a hydrogen-rich companion, and that the GRB has a structured jet geometry in which the gamma rays emerge in a small opening angle within the wider opening angle of the cone of the afterglow. This scenario is able to explain both the spectral line features and the irregular light curve of this afterglow.