TITLE:   GCN CIRCULAR
NUMBER:  45631
SUBJECT: GRB 260917B: Mondy optical upper limit
DATE:    26/09/18 13:51:45 GMT
FROM:    Alina Volnova at IKI RAS <alinusss@gmail.com>

A. Volnova (IKI), E. Klunko (ISTP), A. Pozanenko (IKI), A. Moskvitin (SAO), N. Pankov (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of the GRB 260917B detected by SVOM/ECLAIRs (sb26091701; He et al., GCN 45606) with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) starting on 17 Sep, 2026, at 19:39 UT (i.e., ~16h after the trigger).
The optical counterpart reported previously by SVOM/COLIBRÍ (Dornic et al., GCN 45607) and observed by other facilities (Ma et al., GCN 45610; Yin et al., GCN 45608; Mohan et al., GCN 45611; Jelinek et al., GCN 45617) is not detected in the stacked frame.
Preliminary photometry and observational details are the following:

Date        UT start  t-T0      Exp.    Filter Obj.   Err.  UL      Site/Telescope
                     (mid,days) (n*s)                      (3sigma)
2026-09-17  19:39:51  0.67396   21*120  R      n/d    n/d   21.5    Mondy/AZT-33IK

The photometry (Vega) is based on several nearby PS1-DR2 stars (R-magnitudes Lupton's transformation) and is not corrected for the Galactic extinction.