TITLE: GCN CIRCULAR NUMBER: 42478 SUBJECT: GRB 251023B / EP251023b: Mondy optical upper limit DATE: 25/10/27 13:44:29 GMT FROM: Alina Volnova at IKI RAS <alinusss@gmail.com> A. Volnova (IKI), A. Pozanenko (IKI) , N. Pankov (HSE, IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN: We observed the field of the GRB 251023B / EP251023b (Kang et al., GCN Circ. 42392; Nakahira et al., GCN Circ. 42402; Wu et al., GCN Circ. 42415; Cheung et al., GCN Circ. 42407) in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) starting Oct. 25, 2025, 17:50:36 UT, and taking several frames with 120 s exposures. The optical candidate reported by Mandarakas et al. (GCN Circ. 42404) is not detected in the stacked frame. Photometry and observational details are the following: Date UT start t-T0 Exp. Filter OT Err. UL (mid,days) (n*s) (3sigma) 2025-10-25 17:50:36 2.40386 40*120 R n/d n/d 22.7 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/42478. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...