TITLE: GCN CIRCULAR NUMBER: 6851 SUBJECT: GRB 071003: VLT spectroscopy DATE: 07/10/05 17:30:14 GMT FROM: Paolo D'Avanzo at INAF-OAB D. Fugazza (INAF-OABr), F. Fiore, V. D'Elia (INAF-OAR), P. D'Avanzo (INAF-OABr), S. Piranomonte, L.A. Antonelli (INAF-OAR), G. Chincarini (Univ. Bicocca), S. Covino, G. Tagliaferri (INAF-OABr), M. Della Valle (INAF-OAA), A. Fernandez Soto (Univ. Valencia) report on behalf of the MISTICI collaboration: We observed the optical afterglow of GRB 071003 (Schady et al., GCN 6837; Li et al. GCN 6838; Cenko et al. GCN 6839) with the ESO-Very Large Telescope equipped with the FORS2 camera in spectroscopic mode. We took a 1800s spectrum with the grism 600B (with a resolution of R = 780) on Oct 04.055 UT (~ 0.735 days after the burst), under good seeing conditions (1.0"). The detection of a MgII system at z=1.100 (Perley et al. GCN 6850), cannot be confirmed by our data, because the 2803 AA component of the MgII doublet is contamined by the NaI 5891 and 5897 AA doublet of the nearby bright star. We also find significant lines at 5417 and 5430 AA, which we interpret as the MgII absorption doublet at 2797 and 2803 AA at z=0.937. FeII 2382 is also detected at the same redshift. In addition, we find two more significant absorption lines (5417 and 5446 AA), that can be identified as Ca H and K at z=0.372. Alternatively, the 5446 AA line can be interpreted as the 2803 AA component of the MgII doublet at z=0.942, with the 2797 AA component sharing the absorption feature at 5430 AA with the MgII 2803 of the system at z=0.937. This can be another line-locking example like that of the CIV system of GRB 021004 at z=2.3 (Fiore et al. 2005, ApJ, 624, 853; Starling et al. 2005, MNRAS, 360, 305 ). Finally, we find another MgII doublet at ~4000 AA (z=0.370), not consistent with the Ca system at z=0.372. No emission lines are visible. In summary our analysis of the FORS2 spectrum puts a lower limit on the redshift of GRB 071003 to z=0.937. We thank VLT staff for performing the observations, in particular Thomas Szeifert and Alain Smette.