TITLE:   GCN CIRCULAR
NUMBER:  26553
SUBJECT: GRB 191221B: VLT/X-shooter redshift
DATE:    19/12/22 10:58:41 GMT
FROM:    Maryam Arabsalmani at University of Melbourne  <mar.arabsalmani@gmail.com>

Title: GRB 191221B: VLT/X-shooter redshift

J.-B. Vielfaure (CNRS, Paris Obs.), M. Arabsalmani (ICRAR/UWA), K. E.
Heintz (Univ. Iceland), N. R. Tanvir (Univ. Leicester), A. de Ugarte
Postigo (HETH/IAA-CSIC), D. B. Malesani (DTU Space), J. P. U. Fynbo
(DAWN/NBI), K. Wiersema (Univ. Warwick), S. D. Vergani (CNRS, Paris Obs.),
D. Xu (NAO/CAS), G. Pugliese (Univ. Amsteram), S. Schulze (WIS), D.
Burgarella (AMU, CNRS, CNES, LAM), report on behalf of the Stargate
collaboration:

We observed the optical afterglow of GRB 191221B (Laha et al., GCN 26534;
Longo et al., GCN 26549; Lipunov et al., GCN 26537). using the ESO VLT UT2
(Kueyen) equipped with the X-shooter spectrograph. Our spectra cover the
wavelength range 3000-21000 AA, and consist of 2 exposures by 600 s each.
The observation mid time was 2019 Dec 22.2707 UT (9.843 hr after the GRB).

In a 15 s image taken with the acquisition camera on Dec 22.2592 UT, we
detect the optical afterglow, for which we measure a magnitude r = 17.13 +-
0.01 AB (calibrated against nearby stars from the Skymapper catalog; Wolf
et al. 2018, PASA, 35, 010; https:doi.org/10.4225/41/593620ad5b574).

We detect several absorption profiles in the GRB spectrum including MgII,
MgI, SiII, CIV, AlII, NiII, and FeII lines. From these metal lines we infer
a redshift z=1.148 for the GRB, consistent with the redshift reported by
Swift/UVOT tram (GCN 26538). We also detect the MgII absorption profile
corresponding to an intervening system at z=0.961.

We acknowledge excellent support from the ESO observing staff in Paranal,
in particular Pascale Hibon, Francisco Belmar, and Trystyn Berg.