Spectroscopy of the γ-ray burst GRB 021004:: a structured jet ploughing through a massive stellar wind

We present spectra of the afterglow of the gamma-ray burst GRB 021004 taken with the ISIS spectrograph on the William Herschel Telescope (WHT) and with the Focal Reducer/Low Dispersion Spectrograph 1 (FORS1) on the Very Large Telescope (VLT) at three epochs spanning 0.49-6.62 d after the burst. We observe strong absorption probably coming from the host galaxy, alongside absorption in H I, Si IV and C IV with blueshifts of up to 2900 km s(-1) 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 (WR) star. However, ionization from the burst itself prevents the existence of H I, Si IV and C IV close to the afterglow surface where the fast stellar wind should dominate, and large amounts of blueshifted hydrogen are not expected in a WR star wind. We propose that the WR 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.