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GCN Circular 15645

Subject
GRB 131231A: VLT/X-shooter redshift
Date
2014-01-01T02:36:10Z (10 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@dark-cosmology.dk>
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.
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