TITLE: GCN CIRCULAR NUMBER: 43287 SUBJECT: GRB 260101A: OSIRIS+ redshift z = 2.623 for the new year burst DATE: 26/01/01 03:32:50 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), D. B. Malesani (DAWN/NBI and Radboud), J. F. Agui Fernandez (CAHA), 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), R. Scarpa (GTC) report: We observed the optical counterpart of the new year burst GRB 260101A (Fermi GBM team, GCN 43284, Cenko et al. GCN ) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. In the r-band 30-s acquisition image (beginning on 2026-01-01 at 02:13:40.010 UT, that is 1.286 hr after trigger), the optical afterglow is well detected with a magnitude 18.05 ± 0.10 (AB), calibrated against nearby SDSS objects, and not corrected for Galactic extinction. A total of 2 spectra of 900 s were secured under poor conditions before clouds came in. Indeed, the second spectrum is already affected by a very high background. The first spectrum started on 2026-01-01 at 02:21:06.010 UT (1.410 hr after trigger), using grism R1000B. Continuum is visible over the range 3600-7800 AA. A number of absorption features are detected, which we interpret as due to Ly-alpha, SiII, OI, CII, SiIV, CIV, FeII and AlII, all at a common redshift z = 2.623, which we suggest to be the redshift of GRB 260101A. We note that the Ly-alpha absorption is very weak, making it a rather peculiar sight line. Happy 2026!!! 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/43287. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...