B. Schneider (CEA Paris-Saclay), L. Izzo (DARK/NBI), D. B. Malesani (Univ. Radboud and DAWN/NBI), J. P. U. Fynbo (DAWN/NBI), K. Wiersema (Univ. Lancaster), V. D���Elia (ASI/SSDC, INAF/OAR), K. E. Heintz (Univ. Iceland and DAWN/NBI), report on behalf of the Stargate collaboration:
We observed the afterglow of GRB 211207A (Ambrosi et al., GCN 31182) 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 2021 Dec 8.288 UT (10.04 hr after the GRB).
In a 60 s image taken with the acquisition camera on Dec 8.249 UT, we detect the optical afterglow, for which we measure an AB magnitude r = 22.16 +/- 0.05 mag (calibrated against two nearby stars from the Pan-STARRS catalog).
We clearly detect continuum over the wavelength range 3400 - 18000 AA. A trough is visible around 3960 AA, which we identify as due to H I. From detection of multiple absorption features, which we interpret as due to Si II, O I, C II, Fe II, Si IV, C IV, Al II, Mg II, we infer a redshift z = 2.272. An emission line is also detected at 16390 AA, which we interpret as due to [O III] from the GRB host galaxy.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Joe Anderson, Zahed Wahhaj and Rob van Holstein.