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.