GCN 44877: GRB 260604C: further SAO RAS optical observations

A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI),
A. Volnova (IKI), N. Pankov (IKI) report on behalf of GRB follow-up team

We observed the field of long GRB 260604C discovered by Fermi
(The Fermi GBM team, GCN 44822; Neights & Meegan, GCN 44831);
SVOM (Gotz et al., GCN 44823) and GECAM-B (Luo et al., GCN 44854)
with the Zeiss-1000 1-m telescope of SAO RAS equipped with the CCD
photometer. We obtained 12 x 300 sec. images in Rc band on June 08,
20:01:14--21:07:58 UT.

The OT (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; Gompertz et al., GCN 44837; Gompertz et al., GCN 44843;
Chen et al., GCN 44851; Moskvitin et al., GCN 44852; Sasaki et al.,
GCN 44857; Pankov et al., GCN 44858; Pankov et al., GCN 44862;
Burkhonov et al., GCN 44865; Lincetto et al., GCN 44873)
is detected in our stacked image with the following brightness:

Date       UTstart  t-T0    Exp.   Filter Mag +/- Err.    UL
                    (mid,d) (n*s)                         (3-sigma)  
2026.06.08 20:01:14 4.01102 12*300 Rc     23.08 +/- 0.18  23.8

The preliminary 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.