TITLE: GCN CIRCULAR NUMBER: 42423 SUBJECT: GRB 251022A: COLIBRÍ Further Multi-frequency Optical Observations DATE: 25/10/24 15:05:07 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx> Enrique Moreno Méndez (UNAM), Nikos Mandarakas (LAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), 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), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM): We reobserved the field of the Fermi GRB 251022A (Fermi GBM Team, GCN Circ. 42380; Di Lalla et al., GCN Circ. 42384) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-24 07:25 to 08:30 UTC (from 32.8 to 33.9 hours after the trigger) and obtained 10 minutes of exposure in each of the g, r, i, and z filters and 20 minutes of exposure in the y filter. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In our second epoch, we still detect the optical counterpart firstly reported by GOTO (O’Neill et al. GCN Circ. 42386) and subsequently followed-up by DDOTI (Becerra et al. GCN Circ. 42394), MASTER (Lipunov et al. GCN Circ. 42395), COLIBRÍ (Mandarakas et al. GCN Circ. 42397), and GROWTH (Mohan et al. GCN Circ. 42413) at preliminary magnitudes and upper limits of: g = 22.52 +/- 0.23 r = 22.05 +/- 0.20 i = 21.99 +/- 0.31 z = 21.95 +/- 0.65 y > 20.7 (3-sigma) Compared with the values reported in our first epoch (Mandarakas et al. GCN Circ. 42397), we estimate a temporal decay index of ~-2, consistent with a jet break. Our detection in g suggests that this is not a high-redshift GRB, but instead has z < 3. 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/42423. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...