TITLE: GCN CIRCULAR NUMBER: 42164 SUBJECT: EP250911a: Koshka Zeiss-1000 Optical Upper Limit DATE: 25/10/08 08:38:27 GMT FROM: Alina Volnova at IKI RAS <alinusss@gmail.com> A. Volnova (IKI), A. Novichonok (Petrozavodsk State University, KIAM), I. Nikolenko (INASAN), N. Pankov (HSE, IKI), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN: We observed the localization region of the Fast X-ray Transient EP250911a (Liang et al., GCN 41788; Liang et al., GCN 41809) using the 1-meter Zeiss-1000 telescope of the Simeiz (Koshka) Observatory with the R-band. Observations were performed on Sep. 13, 14, and 16 in the presence of the full Moon. In the stacked frame of the three epochs we did not detect the optical afterglow (Quirola-Vasquez et al., GCN 41790; Angulo et al., GCN 41793; Lai et al., GCN 41801; Evans et al., GCN 41802; Moskvitin & Spiridonova, GCN 41805; Ma et al., GCN 41807). The photometry based on nearby PS1 stars (Lupton's transformation) is the following: Date UT start t-T0 Exp. Filter OT Err. UL (mid, days) (n*s) (3sigma) 2025-09-13 22:44:37 2.60543 45*120 R n/d n/d 21.0 2025-09-14 19:41:15 3.52607 60*120 R n/d n/d 21.7 2025-09-16 20:58:57 5.54397 60*120 R n/d n/d 21.9 2025-09-13+14+16 22:44:37 4.08066 165*120 R n/d n/d 22.6 View this GCN Circular online at https://gcn.nasa.gov/circulars/42164. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...