TITLE: GCN CIRCULAR NUMBER: 45379 SUBJECT: GRB 260808B: GTC/OSIRIS+ spectroscopic redshift z = 1.295 DATE: 26/08/13 20:00:32 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), S. Geier (GTC), M. A. Aloy (UV), C. C. Thoene (AbAO), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), A. Martin-Carrillo (UCD), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), A. Cabrera-Lavers (GTC), A. M. Garcia Rodriguez (GTC) report: We observed the optical counterpart (Turpin et al., GCN 45318; Li et al., GCN 45320), recently reported to be brightening (Li et al., GCN 45359; Guelfand et al., GCN 45364) of GRB 260808B (Bouchet et al., GCN 45316; Brunet et al., GCN 45333) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. In a 30-s acquisition image (beginning on 2026-08-13 at 02:07:19 UT, that is 4.49 days after trigger), the optical afterglow is well detected with a magnitude of 21.70 +- 0.10 (AB), calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. Our measurement confirms the rebrightening (Li et al., GCN 45359; Guelfand et al., GCN 45364). A total of 3 spectra of 1200 s were secured, starting on 2025-08-13 at 02:10:52 UT (mid time 4.51 days after trigger), using grism R1000B. Continuum is visible over the wavelength range 3620-7800 AA. A number of metal absorption features are detected, which we interpret as due to Fe II, Mg II and Mg I, all at a common redshift z = 1.295, which we suggest to be the redshift of GRB 260808B. This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774). View this GCN Circular online at https://gcn.nasa.gov/circulars/45379. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...