TITLE: GCN CIRCULAR NUMBER: 41863 SUBJECT: GRB 250916A: OSIRIS+/GTC redshift z = 2.015 DATE: 25/09/18 10:56:55 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. F. Agui Fernandez (CAHA), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), S. Geier (GTC), L. Izzo (INAF/OACN and DARK/NBI), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), F. Pérez Toledo (GTC) and D. González Gonzalez (GTC) report: We observed the optical afterglow (Belkin et al., GCN 41847) of GRB 250916A (Fermi GBM team, GCN 41839) with OSIRIS+, mounted on the 10.4 m GTC telescope, at Roque de los Muchachos Observatory, on the island of La Palma (Spain). The observation consisted of an r-band acquisition image followed by 3x900 s of spectroscopy using grism R1000B, with a spectral coverage between 3600 and 7800 AA and a resolving power of ~600. The observation started at 2025-09-18 04:15:12.011 UT (1.61 days after the burst onset). In the acquisition image we detect the afterglow at r = 20.0 +/- 0.1 mag (AB), as compared to Pan-STARRS field stars and not corrected by Galactic extinction. The spectrum shows a clear continuum over the entire spectral range. The spectrum continuum shows a moderate curvature, which could be indicative of host galaxy extinction. We detect multiple spectral features that correspond to Lyman alpha, S II, Si II, Si II*, O I, C II, Ni II, Ni II*, Si IV, C IV, Fe II, Fe II*, Al II, Al III, Cr II, Zn II, and Mn II, all at a common redshift of 2.015, which we identify as the redshift of the GRB. We also note the presence of at least one intervening system with C IV features at z = 1.853. View this GCN Circular online at https://gcn.nasa.gov/circulars/41863. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...