TITLE: GCN CIRCULAR NUMBER: 42999 SUBJECT: GRB 251202A / EP251202a: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 40-cm and 1-m telescopes at McDonald Observatory DATE: 25/12/05 12:55:58 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias ipf@iac.es

M. Contreras, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, 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, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).

We report on observations of the optical counterpart of GRB 251202A / EP251202a, detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 42933; and Bala, GCN Circ. 42957), Einstein Probe (EP) (Zhang et al., GCN Circ. 42937; and Liang et al., GCN Circ. 42946), Swift-XRT (Sbarufatti et al., GCN Circ. 42943), AstroSat CZTI (Arya et al., GCN Circ. 42967), Insight-HXMT/HE (Guo et al., GCN Circ. 42970), and SVOM/GRM (Guo et al., GCN Circ. 42997).

We observed the field of GRB 251202A / EP251202a with the Las Cumbres Observatory (LCO) 40-cm telescope and one of the two LCO 1-m telescopes located at the LCO node at McDonald Observatory, Texas. The first observation, with the 40-cm telescope, started on 2025-12-02 at 06:29:14 UT, about 4.67 hours after the Fermi and EP trigger. The optical counterpart first reported by Li et al. (GCN Circ. 42934), at a spectroscopic redshift of z = 2.785 (Zhu et al., GCN Circ. 42939), is clearly detected in our images with the following AB magnitudes, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction:

Date | UT start | mag | error | filter | exposure time (sec) | telescope --------------------------------------------------------------------------------------- 2025-12-02 06:29:14 18.76 0.18 SDSS r' 300 LCO 40-cm 2025-12-02 07:43:09 19.19 0.08 SDSS g' 180 LCO 1-m 2025-12-02 07:46:39 18.91 0.07 SDSS r' 180 LCO 1-m 2025-12-02 07:50:11 18.71 0.08 SDSS i' 180 LCO 1-m

Our results are consistent with other UV and optical observations: Li et al. (GCN Circ. 42934), Lipunov et al. (GCN Circ. 42938), Zhu et al. (GCN Circ. 42939), O’Neill et al. (GCN Circ. 42947), Quirola-Vasquez et al. (GCN Circ. 42948), Kuin and Moss (GCN Circ. 42956), Zheng et al. (GCN Circ. 42964), Ma et al. (GCN Circ. 42966), and Bochenek et al . (GCN Circ. 42977).

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).

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