TITLE: GCN CIRCULAR NUMBER: 42396 SUBJECT: GRB 251013C: ISON-Castelgrande observatory optical observations DATE: 25/10/23 12:56:23 GMT FROM: Alina Volnova at IKI RAS <alinusss@gmail.com> A. Volnova (IKI), S. Schmalz (KIAM), A. Schmalz (KIAM), A. Pozanenko (IKI), N. Pankov (IKI, HSE), F. Graziani (GAUSS Srl) report on behalf of IKI GRB-FuN: We observed field of the GRB 251013C (Fermi GBM team, GCN 42221; Rakotondrainibe et al., GCN 42222) with ORI-22 telescope of ISON-Castelgrande observatory. Observation started on 2025-10-13 (UT) 21:40:38, i.e. ~2 hours after the GRB trigger. The series consists of 165 images with an exposure of 60 s in a Clear filter. The optical afterglow (Palmerio et al., GCN 42223; Perez-Garcia et al., GCN 42225; Konno et al., GCN 42226; Martin-Carrillo et al., GCN 42227; Masi, GCN 42228; Palmerio et al., GCN 42229; Moskvitin et al., GCN 42230; Gompertz et al., GCN 42231; Lipunov et al., GCN 42235; Garnavich, GCN 42240; Watson et al., GCN 42241; López-Cámara et al., GCN 42242; Brosio et al., GCN 42251; Reguitti, GCN 42253; Quintana-Ansaldo et al., GCN 42254; Maksut et al., GCN 42256; Ghosh et al., GCN 42261; Quadri et al., GCN 42262; Adami et al., GCNs 42266, 42276, 42338; Peretto et al., GCN 42267; Ruocco et al., GCN 42269; Calapai, GCN 42275; Globus et al., GCN 42279; Evans, GCN 42281; Abidkhanov et al., GCN 42283; Siegel, GCN 42298; Leonini et al, GCN 42301; Méndez et al., GCN 42318; He et al., GCN 42319; Pavoni et al., GCN 42333; Bochenek and Perley, GCN 42334) is clearly detected, and the preliminary photometry of the stacked frames is the following: Date UT start t-T0 Exp. Filter OT Err. UL (mid,days) (n*s) (3sigma) 2025-10-13 21:40:38 0.17290 15*60 Clear 16.75 0.10 18.0 2025-10-13 21:56:47 0.18487 15*60 Clear 16.86 0.08 18.0 2025-10-13 22:15:08 0.19875 20*60 Clear 17.00 0.10 18.1 2025-10-13 22:37:50 0.21452 20*60 Clear 17.19 0.11 18.1 2025-10-13 22:59:27 0.23325 30*60 Clear 17.47 0.12 18.3 2025-10-13 23:31:46 0.25573 30*60 Clear 17.35 0.13 18.1 2025-10-14 00:04:13 0.28013 35*60 Clear 17.54 0.19 17.9 The photometry is based on several nearby SDSS stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/42396. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...