TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3860 SUBJECT: VLT/UVES spectroscopy of GRB050820 DATE: 05/08/23 10:29:02 GMT FROM: Paul Vreeswijk at ESO C. Ledoux, P. Vreeswijk (ESO), S. Ellison (U. Victoria), A. Jaunsen (ESO/U. Oslo), A. Smette (ESO), J. Fynbo (U. Copenhagen), P. Moller, A. Kaufer (ESO), M. Andersen (AIP), R. Wijers (U. Amsterdam) & J. Hjorth (U. Copenhagen) report: We observed the afterglow (Fox & Cenko, GCN 3829) of GRB 050820 (Page et al, GCN 3830) with UVES at VLT/UT2, starting at 7:08 UT (33 minutes after the burst trigger). The spectra cover most of the optical wavelength range (split into two observations of a total of 60min using one dichroic beam splitter and 40min using the other), with an approximate resolving power of 46,000 (6.5 km/s). We obtain a redshift of z=2.6147 (and therefore confirm the redshift reported by Prochaska et al., GCN 3833) from the detection of numerous host-galaxy metal absorption lines (e.g., CII, CIV, NI, OI, AlII, AlIII, SiII, SiIV, SII, ArI, CrII, FeII, NiII, ZnII) spread over a velocity interval of ~400 km/s. Strong fine-structure lines of CII* and SiII* are also detected. We measure a total neutral hydrogen column density of log N(HI)=21.0 and metallicities of [Si/H]=-0.6 and [Fe/H]=-1.1. In addition, an intervening metal absorption line system with log N(HI)=20.0 is observed at zabs=2.3597. We acknowlegde the excellent support from the ESO staff, and in particular the alertness of night astronomer Stefano Bagnulo.