TITLE: GCN CIRCULAR NUMBER: 18426 SUBJECT: GRB 151021A: Optical counterpart and redshift from X-shooter DATE: 15/10/21 03:53:02 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC A. de Ugarte Postigo, (IAA-CSIC, DARK/NBI), D. Malesani (DARK/NBI), D. Xu (NAOC/CAS) report on behalf of a larger collaboration: We have observed the field of GRB 151021A (Melandri et al., GCN 18425) with X-shooter at the VLT (Paranal Observatory, Chile). Observation started at 02:13 UT (44 min after the burst) and cover the range between 3000 and 24800 AA. In the close proximity of the XRT error circle, we detect a bright, uncatalogued object in the r'-band acquisition image, at coordinates (J2000, 0.5" error): RA = 22:30:34.369 Dec = -33:11:49.45 We suggest that this is the optical afterglow of GRB 151021A and we measure for it R = 18.2 (Vega), derived from comparison with R-band magnitudes for the USNO-B1.0 catalogue. An object at consistent coordinates is also visible in the UVOT white-band image. On a preliminary reduction, the spectrum shows a bright continuum with multiple absorption features, including SII, OI, SiII, SiIV, CIV, AlII, ZnII, CrII, FeII, NiII, MnII, MgII, MgI, as well as fine-structure lines from FeII* and NiII* and Ly-alpha absorption at a common redshift of z = 2.330, which we identify as the redshift of the GRB. We also identify an intervening system at z = 1.490 with features of FeII, MgII, and MgI. We acknowledge the excellent support given by the Paranal staff, and in particular Andrea Mehner, Julien Mili and Jose Velasquez.