TITLE:   GCN CIRCULAR
NUMBER:  45997
SUBJECT: EP261015a: Terskol Zeiss-2000 optical upper limit
DATE:    26/10/20 15:07:34 GMT
FROM:    Alina Volnova at IKI RAS <alinusss@gmail.com>

A. Volnova (IKI), D. Andreev (INASAN), A. Pozanenko (IKI), A. Moskvitin (SAO), N. Pankov (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of Einstein Probe fast X-ray transient EP261015a (Li et al., GCN 45956) with the Zeiss-2000 2-meter telescope of the Terskol Observatory on Oct. 17, 2026, starting at 21:30 UT and taking several exposures in R-band.
The optical counterpart reported previously (Corcoran et al., GCNs 45955, 45972; Fortin et al., GCN 45991) 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)
2026-10-17  21:30:24  2.32810   20*180  R      n/d    n/d   22.8    Terskol/Z2000

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