TITLE: GCN CIRCULAR NUMBER: 42406 SUBJECT: EP251023a: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 40-cm telescope at Teide Observatory DATE: 25/10/23 19:23:09 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es> A. Selezneva, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL). Following the detection of the X-ray transient EP251023a by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (ID: 01709247295, Wu et al., GCN circ. 42388), we observed the field with one of the two Las Cumbres Observatory (LCO) 40-cm telescopes (Planewave Delta Rho 350 telescopes equipped with QHY600 CMOS cameras) located at the LCO node at Teide Observatory (Tenerife, Canary Islands, Spain). The observation, a single exposure of 300 sec in the SDSS r filter, started on 2025-10-23 at 06:09:24 UT, about 3.64 hours after the EP-WXT trigger. The optical counterpart detected by GOTO (GOTO25ixt, Wortley et al., GCN circ. 42387) is clearly detected in our image with a magnitude of r = 18.69 +/- 0.10 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for galactic extinction, that is consistent with other optical detections at similar epochs (Li et al., GCN circ. 42389; Liu et al., GCN circ. 42391) and brighter than the COLIBRÍ detection at a later epoch (Mandarakas et al., GCN circ. 42400). A redshift of z = 2.232 has been measured by Chornock (GCN circ. 42398) using Keck/LRIS. Based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation (LCO program IAC2025B-010). 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/42406. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...