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.