[vsnet-grb-info 20898] GRB 180720B: VLT/X-shooter redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22996
SUBJECT: GRB 180720B: VLT/X-shooter redshift
DATE:    18/07/21 14:34:50 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at dark-cosmology.dk>

P. M. Vreeswijk (Radboud Univ. Nijmegen), D. A. Kann (HETH/IAA-CSIC), K. 
E. Heintz (Univ. Iceland and DAWN/NBI), A. de Ugarte Postigo 
(HETH-IAA/CSIC and DARK/NBI), B. Milvang-Jensen (DAWN/NBI), D. B. 
Malesani (DARK/NBI and DAWN/NBI), S. Covino (INAF/Brera), A. J. Levan 
(Univ. Warwick), G. Pugliese (Amsterdam), report on behalf of the 
Stargate collaboration:

We observed the optical counterpart of GRB 180720B (Siegel et al., GCN  
22973; Martone et al., GCN 22976) with the ESO VLT UT2 equipped with the 
X-shooter spectrograph. Two spectra of 600 s each were secured starting 
on 2018 July 21.384 UT (0.786 days after the GRB), covering the 
wavelength range 3000-25000 AA.

A bright continuum is detected across the entire observed range. Several 
absorption features are detected, which can be identified as due to Fe 
II, Mg II, Mg I, and Ca II, all at z = 0.654. We also identify 
fine-structure transitions due to both Fe II* and Ni II**, thus making 
the redshift association with the GRB secure.

We acknowledge the ESO observing staff at Paranal, in particular Juan 
Carlos Munoz, Emanuela Pompei, and Luca Sbordone.