TITLE: GCN CIRCULAR NUMBER: 12284 SUBJECT: GRB 110818A: Optical afterglow and redshift from VLT/X-shooter DATE: 11/08/19 07:49:47 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst P. D'Avanzo (INAF/Brera), M. Sparre, D. Watson, J. P. U. Fynbo, D. Malesani, B. Milvang-Jensen (DARK/NBI), P. Goldoni (APC/Univ. Paris 7 and SAp/CEA), V. D'Elia (INAF/Rome), and N. R. Tanvir (U. Leicester), report on behalf of the X-shooter GTO GRB collaboration: We observed the field of GRB 110818A (Markwardt et al. GCN 12279) with the ESO VLT equipped with X-shooter. Observations started on 2011-08-19 at 02:47 UT (6.15 hr after the GRB). The acquisition image shows an optical afterglow candidate inside the XRT error circle (Markwardt et al. GCN 12279) at the following coordinates (J2000): RA = 21:09:21.04 Dec = -63:58:52.3 with an uncertainty of about 0.5". The object has a magnitude R ~ 22.3, calibrated assuming R = 19.2 for the USNO-B1 star 0260-0726657 (RA, Dec = 21:09:23.14, -63:58:33.0). A total spectroscopic exposure of 4x1200 s was obtained, covering the spectral range from 300 to 2500 nm. We report a redshift of z = 3.36 based on detection of absorption features from Si II, C IV, Al II, Ca H, Ca K and Mg I. Emission from the [O III] doublet is also observed. We would like to thank the staff at the VLT, in particular, Alain Smette, Patricia Guajardo and Dimitri Gadotti for carrying out the observations.