TITLE:   GCN CIRCULAR
NUMBER:  46018
SUBJECT: EP261023a: Mondy optical upper limit
DATE:    26/10/24 09:07:49 GMT
FROM:    Nicolai Pankov at Independent Researcher <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 the Einstein Probe fast X-ray transient EP261023a (Yang et al., GCN 46014) with the 1.5-m AZT-33IK telescope of Sayan Solar Observatory (Mondy). Observations started on 2026-10-23 at 13:30:00 UT, i.e. ~0.41 days after the EP/WXT trigger. A series of 30x120 s frames was obtained in the R filter.

The optical counterpart (Lei et al., GCN 46015 and 45972) is not detected in the co-add frame from Mondy. Preliminary upper limit and observation details are given below.

Date        UTstart t-T0        Exp.   Filter OT    Err.   UL
                    (mid, days) (s)                       (3sigma)
2026-10-23 13:30:00 0.41437     30x120  R     n/d   n/d    22.2

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