TITLE: GCN CIRCULAR NUMBER: 9761 SUBJECT: GRB 090809 X-shooter redshift DATE: 09/08/10 15:54:48 GMT FROM: Antonio Deugarte at IAA-CSIC D. Malesani, J.P.U. Fynbo (DARK/NBI), L. Christensen (ESO), A. de Ugarte Postigo (OAB-INAF), B. Milvang-Jensen (DARK/NBI), S. Covino (OAB-INAF) report on behalf of a larger collaboration We have observed the afterglow of GRB 090809 (Markwardt et al. GCNC 9754) using the X-shooter spectrograph mounted at the Kueyen telescope of ESO's VLT (Paranal Observatory, Chile) during its science verification phase. The spectrum, consisting of a combined exposure of 4x1800s, was started at 03:40 UT on August 09 (10.15 hrs after the burst). It was obtained with an initial seeing of 0.9" and an airmass of 1.2, when the afterglow had a magnitude of R~21. The spectrum covers the complete range from 3000 to 25000 Angstroms. In a preliminary analysis we identify the Lyman break, Lyman-alpha, SiII, OI, SiI*, SiIV, CIV, FeII, AlII, AlIII and MgII at a common redshift of 2.737 +/- 0.002 (pending final wavelength calibration), which we identify as the redshift of the GRB. We thank the Paranal staff for excellent support, in particular Elena Mason and Claudia Cid.