TITLE:   GCN CIRCULAR
NUMBER:  28840
SUBJECT: GRB 201104B: VLT/X-shooter redshift
DATE:    20/11/05 03:03:00 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

J.-B. Vielfaure (GEPI, Paris Observatory), D. B. Malesani (DTU Space), 
D. A. Kann (HETH/IAA-CSIC), B. Sbarufatti (PSU), N. R. Tanvir (Univ. 
Leicester), and D. H. Hartmann (Clemson University) report on behalf of 
the Stargate Consortium:

We observed the optical afterglow (Marshall et al., GCN 28827) of GRB 
201104B (Sbarufatti et al., GCN 28825) using the ESO VLT UT3 (Melipal) 
equipped with the X-shooter spectrograph. Our spectra cover the 
wavelength range 3000-21000 AA, and consist of 2 exposures of 1200 s 
each. The observation mid-time was 2020 Nov 5.0831 UT (8.431 hr after 
the GRB).

In a 60 s image taken with the acquisition camera on Nov 5.0596 UT, we 
detect the optical afterglow, for which we measure a magnitude r = 21.01 
+/- 0.02 mag (AB magnitude, calibrated against nearby stars from the 
Pan-STARRS catalog; Chambers et al. 2016, arXiv:1612.05560).

We clearly detect continuum over the entire wavelength range. A trough 
is visible around 3590 AA, which we identify as due to H I. From 
detection of multiple absorption features, which we interpret as due to 
SiII, CI, CII, SiIV, CIV, FeII, AlII, AlIII, MgII, we infer a redshift z 
= 1.954. In particular, fine structure lines from Fe II* are visible, 
which confirm the association of the absorption system with the GRB. 
Emission lines are also detected, which we interpret as due to [O II], 
Hbeta, [O III], and Halpha at the same redshift.

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

[GCN OPS NOTE(06nov2020): Per author's request, has two changes:
1) from "which we interpret as due to [O II], [O III], ad Halpha"
to "which we interpret as due to [O II], Hbeta, [O III], and Halpha"
2) adding Ivan Aranda, and Matias Jones to the acknowledgement list.]