TITLE: GCN CIRCULAR NUMBER: 42450 SUBJECT: GRB 251025B: ULL-ASTRO-MASTER optical counterpart detection with IAC80 telescope at Teide Observatory DATE: 25/10/26 03:31:47 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es> J. Hernández Fung, P.G. Berdayes, M. Contreras, B. Gandolfi, L. Juliá-Maroto, A. Schenone-Zanuzzi, E. Urquijo-Rodríguez, J. Basurto Merino, A. Caballero-Almagro, A. Cerón, F. Díaz-Segado, T. Ferrer-Laviña, V. Ghiraldo, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Selezneva, T. Tundidor Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL). Following the detection of GRB 251025B (sb25102502) by SVOM (Hussein et al., GCN circ. 42437), we observed the field with the IAC80 telescope equipped with the CAMELOT2 camera at Teide Observatory (Tenerife, Canary Islands, Spain). The observation, a single exposure of 300 sec in the SDSS g' filter, started on 2025-10-26 at 00:50:31 UT, about 10.44 hours after the SVOM trigger. The optical counterpart detected by Wu et al. (GCN circ. 42438), Gress et al. (GCN circ. 42439), Wu et al. (GCN circ. 42440), and Mohan et al. (GCN circ. 42447) is clearly detected in our image with a magnitude of g = 20.37 +/- 0.12 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for galactic extinction. The optical counterpart position is consistent within the errors with the position of the X-ray afterglow measured by Swift-XRT (Beardmore et al. GCN circ. 42445). This work is based on observations made with the IAC80 telescope operated on the island of Tenerife by the Instituto de Astrofísica de Canarias in the Spanish Observatorio del Teide. We acknowledge the expert assistance of Roberto C. Álvarez and Borja Castañeda. These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain). This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5). View this GCN Circular online at https://gcn.nasa.gov/circulars/42450. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...