TITLE:   GCN CIRCULAR
NUMBER:  44862
SUBJECT: GRB 260604C: Mondy optical observations: evidence of a possible brightening
DATE:    26/06/08 09:03:44 GMT
FROM:    Nicolai Pankov at IKI <colinsergesen@gmail.com>

N. Pankov (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), A. Moskvitin (SAO RAS), 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 AZT-33IK 1.6-m telescope of the Sayan Solar Observatory (Mondy). The observations were conducted starting on 2026-06-07 16:34 UT, i.e. ~2.86 days since GRB. We aquired 30x120 sec exposures in the R-band using a CMOS-photometer.

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; Pankov et al., GCN 44858) is clearly detected in the co-add image and displays an evidence of a brightening in comparison with previous epochs of observations (Moskvitin et al., GCN 44852; Sasaki et al., GCN 44857; Pankov et al., GCN 44858). The observations details and preliminary photometry are summarized below:

Date       UTstart  t-T0    Exp.   Filter Mag    Err.    UL
                    (mid,d) (n*s)                      (3-sigma)
2026-06-07 16:34:49 2.86396 28x120 R      21.18  0.06   23.2

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.