GRB 021004: Tomography of a gamma-ray burst progenitor and its host galaxy

<i>Aims. <i/>We analyse the distribution of matter around the progenitor star of gamma-ray burst GRB 021004 and the properties of its host galaxy with high-resolution echelle and near-infrared spectroscopy.<i>Methods. <i/>Observations were taken by the 8.2 m Very Large Telescope with the Ultraviolet and Visual Echelle spectrograph (UVES) and the Infrared Spectrometer And Array Camera (ISAAC) between 10 and 14 h after the onset of the event.<i>Results. <i/>We report the first detection of emission lines from a GRB host galaxy in the near-infrared, detecting H<i>α<i/> and the [O III] doublet. These allow us to independently measure the systemic redshift (<i>z<i/> = 2.3304 <i>±<i/> 0.0005), which is not contaminated by absorption as the Ly<i>α<i/> line is, and infer the host galaxy properties. From the visual echelle spectroscopy, we find several absorption-line groups spanning a range of about 3000 km s<sup>-1<sup/> in velocity relative to the redshift of the host galaxy. The absorption profiles are very complex with both velocity-broadened components extending over several 100 km s<sup>-1<sup/> and narrow lines with velocity widths of only ~20 km s<sup>-1<sup/>. By analogy with QSO absorption line studies, the relative velocities, widths, and degrees of ionization of the lines (“line-locking”, “ionization-velocity correlation”) show that the progenitor had both an extremely strong radiation field and several distinct mass-loss phases (winds).<i>Conclusions. <i/>These results are consistent with GRB progenitors being massive stars, such as luminous blue variables (LBVs) or Wolf-Rayet stars, providing a detailed picture of the spatial and velocity structure of the GRB progenitor star at the time of explosion. The host galaxy is a prolific star-forming galaxy with a <i>SFR<i/> of ~40 yr<sup>-1<sup/>.

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