TITLE:   GCN CIRCULAR
NUMBER:  25956
SUBJECT: GRB 191004B: VLT/X-shooter redshift
DATE:    19/10/05 08:40:16 GMT
FROM:    Valerio D'Elia at ASDC  <valerio.delia@ssdc.asi.it>

V. D'Elia (SSDC), J. P. U. Fynbo (DAWN/NBI), L. Izzo (DARK/NBI), D. B. Malesani (DTU Space), K. E. Heintz (Univ. of Iceland), N. R. Tanvir (Univ. Leicester), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), S. D. Vergani (CNRS -GEPI/Observatorie de Paris) report on behalf of the Stargate collabaration:
We observed the optical afterglow (Lipunov et al., GCN 25948, Strausbaugh et al., GCN 25949) of the Swift/BAT GRB 191004B (Cenko et al., GCN 25947) using the ESO VLT UT2 (Kueyen) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21000 AA, and consist of 6 exposures by 600 s each. The observation mid time was 2019 Oct 05 at about 5:30 UT (8 hr after the GRB).
In a 30 s image taken with the acquisition camera on Oct 05.1989 UT, we detect the optical afterglow, for which we measure a magnitude r = 21.23 +- 0.03 AB (calibrated against nearby stars from the SkyMapper catalog; Wolf et al. 2018, PASA, 35, 010; https:doi.org/10.4225/41/593620ad5b574).

We clearly detect continuum over the full covered wavelength range. A strong absorption is visible around 5474 AA, and an emission feature at 5479 AA, which we identify as due to H I. This is confirmed by the presence of the Ly alpha forest blueward of this wavelength. We also detect other narrower absorption features which we interpret as due to e.g. C IV and Si IV at a common redshift z = 3.503. The continuum is well detected down to ~800 AA rest frame, i.e, below the Lyman limit.
We also detect an intervening absorber (Mg II, Fe II) at z=2.137
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Camila Navarrete and Bin Yang.