{
  "bibcode": "2015GCN.18426....1D",
  "body": "A. de Ugarte Postigo, (IAA-CSIC, DARK/NBI), D. Malesani (DARK/NBI),\nD. Xu (NAOC/CAS) report on behalf of a larger collaboration:\n\nWe have observed the field of GRB 151021A (Melandri et al., GCN 18425) \nwith X-shooter at the VLT (Paranal Observatory, Chile). Observation \nstarted at 02:13 UT (44 min after the burst) and cover the range between\n3000 and 24800 AA.\n\nIn the close proximity of the XRT error circle, we detect a bright, uncatalogued \nobject in the r'-band acquisition image, at coordinates (J2000, 0.5\" error):\n\nRA = 22:30:34.369\nDec = -33:11:49.45\n\nWe suggest that this is the optical afterglow of GRB 151021A and we measure \nfor it R = 18.2 (Vega), derived from comparison with R-band magnitudes for the \nUSNO-B1.0 catalogue. An object at consistent coordinates is also visible in the \nUVOT white-band image.\n\nOn a preliminary reduction, the spectrum shows a bright continuum with multiple \nabsorption features, including SII, OI, SiII, SiIV, CIV, AlII, ZnII, CrII, FeII, NiII, MnII, \nMgII, MgI, as well as fine-structure lines from FeII* and NiII* and Ly-alpha \nabsorption at a common redshift of z = 2.330, which we identify as the redshift of \nthe GRB. We also identify an intervening system at z = 1.490 with features of \nFeII, MgII, and MgI.\n\nWe acknowledge the excellent support given by the Paranal staff, and in \nparticular Andrea Mehner, Julien Mili and Jose Velasquez.",
  "circularId": 18426,
  "createdOn": 1445399582000,
  "email": "deugarte@iaa.es",
  "subject": "GRB 151021A: Optical counterpart and redshift from X-shooter",
  "submitter": "Antonio de Ugarte Postigo at IAA-CSIC  <deugarte@iaa.es>",
  "eventId": "GRB 151021A"
}