TITLE:   GCN CIRCULAR
NUMBER:  28847
SUBJECT: GRB 201103B (ZTF20acozryr/AT2020yxz): VLT/X-shooter redshift
DATE:    20/11/06 04:58:38 GMT
FROM:    Dong Xu at NAOC/CAS  <dxu@nao.cas.cn>

D. Xu (NAOC), J.-B. Vielfaure (APC, Paris University), D. A. Kann 
(HETH/IAA-CSIC), D. B. Malesani (DTU Space), D. A. Perley (LJMU), A. J. 
Levan (Radboud U. Nijmegen), S. Klose, B. Stecklum (both TLS Tautenburg),
K. Wiersema (Univ. Warwick), and A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI) 
report on behalf of the Stargate Consortium:

We observed the transient ZTF20acozryr/AT2020yxz (Coughlin et al., GCN 
28841), which has been proposed to be the optical afterglow of GRB 
201103B (Svinkin et al., GCN 28844) using the ESO VLT UT3 (Melipal) 
equipped with the X-shooter spectrograph. Our spectra cover the 
wavelength range 3000-21000 AA, and consist of 4 exposures of 1200 s 
each. The observation mid-time was 2020 Nov 6.116 UT (56.66 hr/2.361 
days after the GRB).

In a 60 s image taken with the acquisition camera on Nov 6.0771 UT, we 
detect the optical afterglow, for which we measure a preliminary 
magnitude r ~ 20.7 mag (AB magnitude, calibrated against a nearby galaxy 
from the Pan-STARRS catalog; Chambers et al. 2016, arXiv:1612.05560).

We clearly detect continuum over the entire wavelength range. From 
detection of multiple absorption features, which we interpret as due to 
CII, MgII, and multiple FeII lines, we infer a redshift z = 1.105. We 
suggest this to be the redshift of the transient, which we hereby 
confirm spectroscopically to be the afterglow of GRB 201103B.

We acknowledge excellent support from the ESO observing staff in Paranal,
in particular Ivan Aranda and Matias Jones.

[GCN OPS NOTE(04nov2020): Per author's request, Ivan Aranda and Matias Jones
were added to the last sentence.]
[GCN OPS NOTE(07nov20): Per author's request, Stecklum was added
to the author's list.]