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.