TITLE: GCN CIRCULAR NUMBER: 42095 SUBJECT: GRB 251003A: COLIBRÍ photometric redshift DATE: 25/10/03 15:49:16 GMT FROM: Benjamin Schneider at MIT <bschn@mit.edu> Benjamin Schneider (LAM), Ny Avo Rakotondrainibe (LAM), Enrique Moreno Méndez (UNAM), Antonio de Ugarte Postigo (LAM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), 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), Margarita Pereyra (UNAM) and Fredd Sánchez Álvarez (UNAM): COLIBRÍ performed additional observations of the Fermi and Swift GRB 251003A (Fermi GBM team, GCN Circ. 42069; Beardmore et al., GCN Circ. 42070) starting from 2025-10-03 04:04:59 (2.1 hr after the trigger) and obtained images in the g, i, y filters. The photometry of the afterglow (Aceituno et al. GCN Circ. 42072; Moreno Méndez et al. GCN Circ. 42073; Strausbaugh et al. GCN Circ. 42074; Freeberg et al. GCN Circ. 42085; Worssam et al., GCN Circ. 42094) was performed using STDWeb/STDPipe (Karpov 2025), was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. After combining data obtained from Moreno Méndez et al., GCN Circ. 42073 and correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.36 mag (Schlafly & Finkbeiner 2011), we fit a power-law model to the grizy-bands using the SMC extinction curve and we derive a photometric redshift of z = 4.38+0.07-0.04 (1-sigma c.l., statistical errors only). Our result is consistent with the GTC/OSIRIS+ redshift of z = 4.41 (Sanchez-Ramirez et al., GCN Circular 42086). 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/42095. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...