TITLE: GCN CIRCULAR NUMBER: 33281 SUBJECT: GRB 230204B: VLT/X-shooter redshift DATE: 23/02/06 11:18:58 GMT FROM: Daniele B Malesani at Radboud U A. Saccardi (GEPI, Paris obs.), D. A. Kann (Goethe Univ.), J. Palmerio (GEPI, Paris obs. and IAP), V. D’Elia (SSDC and INAF-OAR), B. Schneider (MIT), L. Izzo (DARK/NBI), A. de Ugarte Postigo (OCA), D. B. Malesani (Radboud Univ. and DAWN/NBI) report on behalf of the Stargate collaboration: We observed the optical afterglow of GRB 230204B (Serino et al., GCN 33265; Swain et al., GCN 33269), also known as AT 2023bic (Smartt et al., GCN 33278), using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-25000 AA, and consist of 4 exposures by 600 s each. The observation mid time was 2023 Feb 6.22 UT (31.4 hr after the GRB). In a 30 s image taken with the acquisition camera on Feb 6.19 UT, we detect the optical afterglow, for which we measure a magnitude r = 21.55 +- 0.18 AB (calibrated against a single nearby star from the Pan-STARRS catalog). A faint continuum is the detected in the visible and near-infrared arms. Several, weak absorption features can be identified, which we interpret as due to Mg II, Mg I and Fe II at a common redshift of z = 2.142. While individual lines have low S/N, the combined detection of multiple features provides a convincing measurement of the redshift of this absorption system. The association with the PGC1 0045721 galaxy group is thus a chance superposition, as already suggested by Smartt et al. (GCN 33278). We acknowledge excellent support from the ESO observing staff in Paranal, in particular Claudia Paladini and Heidi Korhonen.