TITLE: GCN CIRCULAR NUMBER: 42009 SUBJECT: GRB 250920C: ULL-ASTRO-MASTER early detection of the optical afterglow with LCO 40-cm telescope at Siding Spring Observatory DATE: 25/09/26 19:40:45 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias ipf@iac.es

V. Ghiraldo, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, 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, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).

We report on a Las Cumbres Observatory (LCO) 40-cm telescope early observation of the long-duration Gamma-Ray Burst GRB 250920C, detected by Fermi (Fermi GBM team, GCN circ. 41903; and Mailyan et al., GCN circ. 41917), Swift (Gupta et al., GCN circ. 41904; Kuin et al., GCN circ. 41920; Goad et al., GCN circ. 41922; Gupta et al., GCN circ. 41924; and D'Ai et al. GCN circ. 41935), SVOM (Wang et al., GCN circ. 41929), and Konus-Wind (Panteleeva et al., GCN circ. 41957).

We observed the field of GRB 250920C with one of the two LCO 40-cm telescopes (Planewave Delta Rho 350 telescopes equipped with CMOS QHY600 cameras) located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 300 sec in the SDSS r filter, started on 2025-09-20 at 17:21:51 UT, about 1.94 hours after the Fermi and Swift trigger. The optical and near-infrared counterpart reported first by Wortley et al. using GOTO (GCN circ. 41907) and with observations by several other groups (GCN circ. 41911, GCN circ. 41913, GCN circ. 41916, GCN circ. 41920, GCN circ. 41925, GCN circ. 41928, GCN circ. 41943, GCN circ. 41947, GCN circ. 41951, GCN circ. 41952, GCN circ. 41955, GCN circ. 41956, GCN circ. 41965, and GCN circ. 41967) is clearly detected in our image, with a magnitude of r = 18.77 +/- 0.10, calibrated against PanSTARRS-1 DR2 and not corrected for galactic extinction, that is consistent with another reported r-band detection at a similar epoch.

A redshift of z = 1.399 - 1.40 has been measured by Sánchez-Ramírez et al. (GCN circ. 41928), de Wet et al. (GCN circ. 41952), and Izzo et al. (GCN circ. 41955).

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 (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 (Tenerife, Canary Islands, Spain).

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