TITLE: GCN CIRCULAR NUMBER: 38822 SUBJECT: GRB 250103B: LCO/1m optical afterglow candidate detection DATE: 25/01/04 15:54:30 GMT FROM: Damien Turpin at CEA-Saclay dturpin-astro@hotmail.com

D. Turpin (CEA/Irfu), A. Saccardi (GEPI/Obs. de Paris, CEA/Irfu), S. Basa (UAR Pytheas, OHP, LAM), D. B. Malesani (DAWN/NBI and Radboud Univ.), J. T. Palmerio (CEA/Irfu, GEPI/Obs. de Paris), S. Vergani (GEPI/Obs. de Paris), E. Le Floc'h (CEA Paris-Saclay, DAp/AIM) on behalf of a larger collaboration.

We observed the field of GRB 250103B detected by Swift (Eyles-Ferris et al. GCN 38796) and SVOM (Bouchet et al., GCN 38797) with the LCO 1.0m telescope at the Sutherland Observatory (SAAO) equipped with the Sinistro instrument. Our observation started at 2025-01-03T20:25:20.617 (T - TGRB ~ 4.68 hr) with 5 x 120s exposure using the sdss-r filter.

We marginally detect the optical afterglow candidate seen by SVOM/VT (Qiu et al., GCN 38813) and VLT (Habeeb et al., GCN 38820) from the difference image analysis between our stacked image and the Legacy Survey DR10 template image. At the position reported by VLT, we derived the following magnitude r = 22.32 +/- 0.26 (at Tmid - T0 = 4.78 hr) calibrated with the SkyMapper DR4 catalog and not corrected from the galactic extinction E(B-V): 0.01.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004719.

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