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.