TITLE: GCN CIRCULAR NUMBER: 44858 SUBJECT: GRB 260604C: Simeiz Zeiss-1000 optical observations of the afterglow DATE: 26/06/07 19:31:45 GMT FROM: Nicolai Pankov at IKI <colinsergesen@gmail.com> N. Pankov (IKI), A. Pozanenko (IKI), A. Novichonok (KIAM), A. Moskvitin (SAO RAS), I. Nikolenko (INASAN), A. Volnova (IKI) report on behalf of IKI-GRB-FuN: We observed the field of GRB 260604C (The Fermi GBM team, GCN 44822; Neights & Meegan, GCN 44831; Gotz et al., GCN 44823) with the Zeiss-1000 1-m telescope of the Simeiz Observatory (mnt. Koshka) of INASAN. The observations were conducted on 2026-06-05 and 2026-06-06 epochs using a CCD-photometer and the R-filter. The optical afterglow (Lipunov, GCN 44827; Konno et al., GCN 44828; GCN 44836; Zhu et al., GCN 44832; Becerra et al., GCN 44834; Hellot et al., GCN 44835; Konno et al., GCN 44836; Gompertz et al., GCNs 44837, 44843; Chen et al., GCN 44851; Moskvitin et al., GCN 44852; Sasaki et al., GCN 44857) is clearly detected in the co-add images obtained on both epochs. The observations details and preliminary photometry are summarized below: Date UTstart t-T0 Exp. Filter Mag Err. UL (mid,d) (n*s) (3-sigma) 2026-06-05 21:31:13 1.07291 26x150 R 21.35 0.14 22.4 2026-06-06 20:09:21 2.03341 46x150 R 21.53 0.19 22.3 The photometry is based on nearby SDSS stars (R-magnitudes were obtained using Lupton 2005 transformation equations) as in Moskvitin et al., GCN 44852, and was not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44858. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...