TITLE: GCN CIRCULAR NUMBER: 42855 SUBJECT: GRB 251126A: COLIBRÍ photometric redshift z = 3.5 DATE: 25/11/27 04:57:59 GMT FROM: Camila Angulo Valdez at UNAM <camiangulo@astro.unam.mx> Camila Angulo (UNAM), Ny Avo Rakotondrainibe (LAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the Swift GRB 251126A (Caputo et al., GCN Circ. 42843) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-11-27 02:49 to 04:14 UTC (from 7.6 to 9.1 hours after the trigger) and obtained, respectively, 10, 27, 11, 23, and 23 minutes of exposure in the g, r, i, z, and y filters. The data were reduced, coadded, and analyzed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We detected the optical counterpart reported by Swain et al. (GCN Circ. 42844), Lipunov et al. (GCN Circ. 42847), Fu et al. (GCN Circ. 42848), Reguitti et al. (GCN Circ. 42849), and Broens et al. (GCN Circ. 42850) at preliminary magnitudes of: r = 20.62 +/- 0.05 z = 19.86 +/- 0.07 In our data, we see a clear break in the spectrum in g. After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.063 mag (Schlafly & Finkbeiner 2011) and fitting a power-law model to the grizy-bands with SMC extinction, we derive a photometric redshift of z = 3.52 (+0.18, -0.20, 1-sigma confidence interval). Further observations are ongoing. We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/42855. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...