TITLE: GCN CIRCULAR NUMBER: 18187 SUBJECT: GRB 150821A: VLT/X-shooter redshift DATE: 15/08/21 11:30:04 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst V. D'Elia (ASI/ASDC and INAF-OAR), T. Kruehler (ESO), K. Wiersema (U. Leicester), N. R. Tanvir (U. Leicester), J. Japelj (U. Ljubljana), G. Pugliese (API/UvA), D. Malesani (DARK/NBI), B. Milvang-Jensen (DARK/NBI), J. P. U. Fynbo (DARK/NBI), report on behalf of a larger collaboration: We observed the optical counterpart of GRB 150821A (Troja et al., GCN 18186) with the ESO VLT equipped with the X-shooter spectrograph. Four spectra with individual exposure time of 600 s were secured covering the wavelength range 3,000-25,000 AA. Observations were carried out in robotic "rapid response mode" (RRM) and spectroscopy began 12.4 min after the BAT trigger (7.7 min after the XRT position, at which point the VLT was triggered). The afterglow is well detected in our acquisition image, with a magnitude of R ~ 16 (11.1 min after the GRB), calibrated to one nearby USNO star. We detect a number of absorption features at a common redshift of z = 0.755, including Mg II, Mg I, Fe II, Zn II. We also detect fine-structure lines at the same redshift (e.g. FeII* 2396 / 2389, and Ni II** 2166 / 2217 / 2223), confirming that the absorption system is local to the GRB and not intervening. We thank the kind assistence of the observing staff at Paranal, in particular Claudia Cid and Marcela Espinoza.