TITLE: GCN CIRCULAR NUMBER: 42773 SUBJECT: EP251118a / GRB 251118C: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 1-m telescope at CTIO DATE: 25/11/19 23:38:46 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es> E. Urquijo-Rodríguez, 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, A. Selezneva, T. Tundidor Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL). Following the detection of the fast X-ray transient EP251118a by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Jiang et al., GCN circ. 42749) and by the EP Follow-up X-ray Telescope (FXT) (Shi et al., GCN circ. 42754), we observed the field with one of the three Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Cerro Tololo Inter-American Observatory (CTIO), Chile. The observation, a single exposure of 500 sec in the SDSS r' filter, started on 2025-11-19 at 07:58:16 UT, about 15.26 hours after the EP-WXT trigger. The optical counterpart first detected by NOT ALFOSC (Malesani et al., GCN 42751), at a redshift of z = 1.216 (An et al., GCN circ. 42756) is clearly detected in our image with a magnitude of r' = 20.69 +/- 0.06 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Our result is consistent with other optical and near-infrared observations: Belkin et al. (GCN circ. 42758), Yadav et al. (GCN circ. 42760), Busmann et al. (GCN circ. 42763), and Francile et al. (GCN circ. 42767). EPP251118a is likely associated with the long-duration GRB 251118C detected by Konus-Wind (Frederiks et al., GCN circ. 42765), SVOM/GRM (Wang et al., GCN circ. 42766), Swift/BAT (DeLaunay et al., GCN circ. 42770),and Glowbug (Woolf et al., GCN circ. 42771). This work makes use of observations from the Las Cumbres Observatory global telescope network (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/42773. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...