TITLE: GCN CIRCULAR NUMBER: 44819 SUBJECT: GRB 260603A: Simeiz Zeiss-1000 optical upper limit DATE: 26/06/04 09:46:23 GMT FROM: Nicolai Pankov at IKI <colinsergesen@gmail.com> N. Pankov (IKI), A. Pozanenko (IKI), A. Novichonok (KIAM), A. Volnova (IKI) report on behalf of IKI-GRB-FuN We observed the field of GRB 260603A (Zhao et al., GCN 44787; Li et al., GCN 44805) with the Zeiss-1000 1-meter telescope of Simeiz Observatory starting on 2026-06-03 UT 19:38:36 (~0.6 days since trigger). A series of 20x180 sec exposures was acquired in the R-filter using a CCD-photometer. We do not detect the optical afterglow (Guelfand et al., GCN 44791, 44797; Li et al., GCN 44799; Freeberg et al., GCN 44800). The preliminary upper limit and the observation details are summarized below: Date UTstart t-T0 Exp. Filter Mag Err. UL (mid,days) (n*s) (3-sigma) 2026-06-03 19:38:36 0.60443 20*180 R n/d n/d 22.3 The photometry is based on nearby USNO-B1.0 stars and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44819. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...