TITLE:   GCN CIRCULAR
NUMBER:  44803
SUBJECT: GRB 260601B / EP260601a: Mondy optical upper limit
DATE:    26/06/03 13:30:59 GMT
FROM:    Alina Volnova at IKI RAS <alinusss@gmail.com>

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

We observed the field of the fast X-ray transient GRB 260601B / EP260601a (Yang et al., GCN 44766; Yu et al., GCN 44767; Wang et al., GCN 44768, 44785; Waratkar et al., GCN 44771; Sugita et al., GCN 44773; Cheung GCN 44801) with the AZT-33IK telescope of the Mondy observatory starting on 2026-06-02 UT 15:45:12 and taking several 120-second exposures in the R-band. In the sdtacked frame we do not detect the optical counterpart reported previously (Liu et al., GCN 44764; Guelfand et al., GCN 44765; Mo et al., GCN 44777; Liu et al., GCN 44789 ).
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)
2026-06-02  15:45:12  0.87559   20*120  R      n/d    n/d   21.1    Mondy/AZT-33IK

The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction.