TITLE: GCN CIRCULAR NUMBER: 44368 SUBJECT: GRB 260421B: VLT/X-shooter spectroscopic redshift z = 2.115 DATE: 26/04/21 06:41:33 GMT FROM: Benjamin Schneider at MIT <bschn@mit.edu> B. Schneider (LAM), A. Saccardi (CEA/Irfu), N. Habeeb (Leicester), R. Brivio (INAF/OAB), A. L. Thakur (INAF/IAPS), D. B. Malesani (DAWN/NBI and Radboud), report on behalf of the Stargate collaboration: We observed the optical counterpart (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366) of the SVOM and Fermi GRB 260421B (Zhao et al., GCN 44362; Fermi GBM Team, GCN 44360) 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 of 600 s each. The observation mid time was 05:39 UT on 2026 April 21 (1.42 hr after the SVOM/ECLAIRs trigger). In a 5-s acquisition image (taken 61 min after the SVOM/ECLAIRs trigger), the afterglow is detected with a magnitude r = 20.43 +/- 0.05 AB, calibrated against the single Pan-STARRS object lying in our field of view, and not corrected for Galactic extinction. In a preliminary reduction of the spectra, we detect a continuum over the entire wavelength range. At the blue edge of the spectrum, we identify a broad trough at around 3786 AA, likely due to H I Lyman-alpha. Multiple absorption lines are also detected, which we interpret as due to, among others, S II, Si II, O I, Si II*, C II, C II*, Si IV, C IV, Fe II, Fe II*, Mg II, all at a common redshift z = 2.115. We consider this to be the redshift of GRB 260421B. We acknowledge the excellent expert support from the observing staff in Paranal, in particular Francisco Caceres and Matias Jones. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495). We thank the COLIBRÍ (FM-GFT) team for alerting us of the afterglow prior to their GCN release. View this GCN Circular online at https://gcn.nasa.gov/circulars/44368. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...