TITLE: GCN CIRCULAR NUMBER: 13051 SUBJECT: GRB 120311A: VLT/X-shooter observations DATE: 12/03/12 19:24:15 GMT FROM: Paul Vreeswijk at U of Iceland Paolo D'Avanzo (INAF/OAB), Bo Milvang-Jensen (DARK/NBI), Paul Vreeswijk (U. Iceland), Nial Tanvir (U. Leicester), Daniele Malesani (DARK/NBI), Paolo Goldoni (APC/U. Paris 7 and SAp/CEA), Lex Kaper (U. Amsterdam), Stefano Covino (INAF/OAB) and Johan Fynbo (DARK/NBI) report on behalf of a larger collaboration: The optical afterglow (Melandri et al., GCN 13033) of the Swift GRB 120311A (Markwardt et al., GCN 13032; Evans et al., GCN 13034) was observed spectroscopically with VLT/X-shooter, starting on 2012 March 11.384 UT (3.65 hours after the GRB trigger). An exposure of 2400 seconds was secured, which ran well into morning twilight. The approximate X-shooter wavelength range is 0.3-2.5 micron. The seeing during the observations was about 0.8" in the optical, and the spectrum has a resulting resolving power of 5000 in the ultraviolet and near-infrared arms, and around 9000 in the optical. The reduced spectrum shows a very weak continuum in all three arms. The very low signal-to-noise ratio prevents us from identifying any significant absorption or emission lines. From the detection of continuum flux down to roughly 3700 Angstrom we set an upper limit to the GRB 120311A redshift of approximately z < 3. Using acquisition images preceding the spectrum, with a mid-exposure time of March 11.373 UT (3.40 hours after the GRB), we measure R = 21.57 +- 0.05 for the afterglow, assuming R=18.30 for the USNO-B1.0 star 1042-0333539 at RA = 18:12:21.82, Dec = +14:17:24.6. We are grateful for the support of the ESO staff at Paranal, in particular that of Gabriel Brammer and Stan Stefl.