A flash in the dark: UVES very large telescope high-resolution spectroscopy of gamma-ray burst afterglows

We present the first high-resolution (R = 20,000-45,000, corresponding to 14 km s^-1 at 4200 A to 6.6 km s^-1 at 9000 Å) observations of the optical afterglow of gamma-ray bursts. GRB 020813 and GRB 021004 were observed by UVES at the Very Large Telescope 22.19 and 13.52 hr after the trigger, respectively. These spectra show that the interstellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spans a total velocity range of up to about 3000 km s^-1. Several narrow components are resolved down to a width of a few tens of km s^-1. In the case of GRB 021004 we detected both low- and high-ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.