We present spectra of the optical transient of GRB 021004 obtained with the Hobby-Eberly Telescope starting 15.48 hr, 20.31 hr, and 4.84 days after the γ-ray burst and a spectrum obtained with the H. J. Smith 2.7 m Telescope starting 14.31 hr after the γ-ray burst. GRB 021004 is the first burst afterglow for which the spectrum is dominated by absorption lines from high-ionization species with multiple velocity components separated by up to 3000 km s-1. We argue that these absorption lines are likely to come from shells around a massive progenitor star. The high velocities and high ionizations arise from a combination of acceleration and flash ionization by the burst photons and the wind velocity and steady ionization by the progenitor. We also analyze the broadband spectrum and the light curve so as to distinguish the structure of gas within 0.3 pc of the burster. We delineate six components in the medium surrounding the γ-ray burst along the line of sight: (1) The z ≅ 2.293 absorption lines arise from the innermost region closest to the burst, where the ionization will be highest and the 3000 km s-1 velocity comes from the intrinsic velocity of a massive star wind boosted by acceleration from the burst flux. For a mass-loss rate of ∼ 6 × 10-5 M⊙ yr-1, this component also provides the external medium with which the jet collides over radial distances 0.004-0.3 pc to create the afterglow light. (2) A second cloud or shell produces absorption lines with a relative velocity of 560 km s -1. This component could be associated with the shell created by the fast massive star wind blowing a bubble in the preceding slow wind at a radial distance on the order of 10 pc or by a clump at ∼0.5 pc accelerated by the burst. (3) More distant clouds within the host galaxy that lie between 30 and 2500 pc and have been ionized by the burst will create the z ≅ 2.33 absorption lines. (4-6) If the three bumps in the afterglow light curve at 0.14, 1.1, and 4.0 days are caused by clumps or shells in the massive star wind along the line of sight, then the radii and overdensities of these are 0.022, 0.063, and 0.12 pc and 50%, 10%, and 10%, respectively. The immediate progenitor of the γ-ray burst could be either a WC-type Wolf-Rayet star with a high-velocity wind or a highly evolved massive star the original mass of which was too small for it to become a WN-type Wolf-Rayet star. In summary, the highly ionized lines with high relative velocities most likely come from shells or clumps of material close to the progenitor, and these shells were plausibly produced by a massive star soon before its collapse.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science