TITLE: GCN CIRCULAR NUMBER: 9012 SUBJECT: GRB 090313: VLT spectroscopy DATE: 09/03/19 22:51:47 GMT FROM: Paolo D'Avanzo at INAF-OAB C. C. Thoene (INAF-OAB), P. D'Avanzo (Univ. Bicocca & INAF-OAB), D. Fugazza (INAF-OAB), A. de Ugarte Postigo (ESO), V. D'Elia, S. Piranomonte (INAF-OAR), S. Covino, S. Campana (INAF-OAB), G. Chincarini (Univ. Bicocca) report, on behalf of the MISTICI collaboration: We observed the optical afterglow of GRB 090313 (Chornock et al. GCN 8979; Mao et al. GCN 8980) with the ESO VLT about 1.9 days after the burst with the FORS2 camera in spectroscopic mode. We took 4 exposures of 20 min each with the 600I grism covering the range 6800-9300 Angstrom with a resolution of R=1500. While we confirm the redshift reported by Chornok et al. (GCN 8994), we also report the detection of a broad double absorption feature around observed lambda 7830 and 7840 angstrom that does not fit any transition at the GRB redshift (z = 3.375). This double feature can be interpreted as Mg II 2796,2803, each consisting of 2 velocity components with a separation of about 300 km/s, and could be a signature of an intervening system at z ~ 1.8. We cannot confirm the presence of Mg I 2852 at z = 1.8 due to the possible overlap with Mg I 1827 absorption at the redshift of the GRB. We acknowledge excellent support from the ESO staff, in particular Yuri Beletsky.