TITLE: GCN CIRCULAR NUMBER: 43008 SUBJECT: GRB 251205A: GTC/OSIRIS+ redshift z = 1.100 DATE: 25/12/06 09:08:39 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> A. de Ugarte Postigo (LAM), S. Geier (GTC), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), L. Izzo (INAF/OACN and DARK/NBI), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), D. Perez Valladares (GTC) report: We observed the optical counterpart of GRB 251205A (Lanava et al., GCN 43005; Lipunov et al., GCN 43006) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. In the 30-s acquisition image (beginning on 2025-12-06 at 05:21:34 UT, that is 5.68 hr after trigger), the optical afterglow is well detected with a magnitude i = 17.78 +- 0.03 AB, calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. We also report the following, improved coordinates (0.3" accuracy): RA(J2000) = 13:21:05.32 Dec(J2000) = +29:55:43.6 A total of 4 spectra by 900 s were secured, starting on 2025-12-06 at 05:31:22 UT (5.86 hr after trigger), using grism R1000B. Continuum is detected over the range 3750-7750 AA. A number of absorption features are detected, which we interpret as due to Fe II 2344, 2383, 2586, 2600, Mg II 2796, 2803, and Mg I 2852, all at a common redshift z = 1.100, which we suggest to be the redshift of GRB 251205A. Given the redshift value measured in absorption, it is thus unlikely that the GRB is associated with the nearby, bright galaxy with photometric redshift z = 0.25 noted by Lipunov et al. (GCN 43006). View this GCN Circular online at https://gcn.nasa.gov/circulars/43008. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...