TITLE: GCN CIRCULAR NUMBER: 42468 SUBJECT: EP251023a: Mondy optical observations DATE: 25/10/27 09:21:37 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 EP transient EP251023a (Wu et al., GCN 42388) in the R-filter with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy). We observed the field on Oct. 23 and 25, 2025, taking several frames with 120 s exposures. The optical couterpart detected by GOTO (Wortley et al., GCN 42387) and also observed by Li et al., GCN 42389; Liu et al., GCN 42391; Mandarakas et al., GCN 42400; Lipunov et al., GCN 42405; Selezneva et al., GCN 42406; Mohan et al., GCN 42411; Rakotondrainibe et al., GCN 42420; Rajabov et al., 42422, is clearly detected in the stacked frame of the first epoch and is not detected in that of the second one. The preliminary photometry is the following: Date UT start t-T0 Exp. Filter OT Err. UL (mid,days) (n*s) (3sigma) 2025-10-23 20:15:41 0.75316 20*120 R 20.64 0.12 22.0 2025-10-25 20:07:45 2.75807 35*120 R n/d n/d 22.7 The photometry is based on several nearby PS1 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/42468. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...