TITLE: GCN CIRCULAR NUMBER: 15645 SUBJECT: GRB 131231A: VLT/X-shooter redshift DATE: 14/01/01 02:36:10 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst D. Xu (DARK/NBI), D. Malesani (DARK/NBI), N. R. Tanvir (Univ. Leicester), V. D'Elia (ASI/ASDC), A. de Ugarte Postigo (IAA/CSIC and DARK/NBI), T. Kruehler (ESO), J. P. U. Fynbo (DARK/NBI) report on behalf of a larger collaboration: We observed the optical counterpart of the Fermi/LAT GRB 131231A (Sonbas et al., GCN 15640; Jenke & Xiong, GCN 15644; Xu et al., GCN 15641) with the ESO VLT equipped with the X-shooter spectrograph. The mean time of our observation is 2014 Jan 1.04 UT, that is 20.2 hr after the GRB. The spectrum covers the wavelength range 3200 to 18,000 AA. The trace of the afterglow is well visible in all arms. We detect several absorption features which we interpret as due to Fe II, Mg II, Ca H and K, all at a common redshift z = 0.642 (wavelength solution based on archival calibration lamps). Emission lines corresponding to the [O III] doublet are also detected at the same redshift. We acknowledge excellent support from the observing staff in Paranal, in particular Steffen Mieske.