TITLE:   GCN CIRCULAR
NUMBER:  40451
SUBJECT: EP250511a: Koshka Zeiss-1000 Optical Upper Limit
DATE:    25/05/13 10:27:04 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>

N. Pankov (HSE, IKI), A. Novichonok (Petrozavodsk State University, KIAM), I. Nikolenko (INASAN), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of EP250511a (Lian et. al, GCN 40429; Li et. al, GCN 40434) using the 1-meter Zeiss-1000 telescope of the Simeiz (Koshka) Observatory. The R-band observations began on 2025-05-11 19:37:13 UT, i.e. ~0.51 days since trigger at a low altitude (~40 deg) and in the presence of the full Moon. The optical counterpart (Schneider et. al, GCN 40431; Li et. al, GCN 40433; Sanchez-Sierras et. al, GCN 40449) is not detected in the stacked image of an exposure of 86*60 sec. The preliminary upper limit is given below:

Date       UT start  t-T0         Exp.    Filter   OT   Err.     UL
                     (mid, days)  (n*s)                       (3sigma)
2025-05-11 19:37:13  0.51405      86*60     R      n/d  n/d     18.9

The photometry was calibrated using nearby stars from USNO-B1.0 and has not been corrected for the Galactic extinction. Our result is consistent with non-detections reported by (Lipunov et. al, GCN 40432; Brivio et. al, GCN 40435; Zhong et. al, GCN 40442; Busmann et. al, GCN 40445).

Ref. stars
USNO-B1.0
RA Dec R2
202.1106 -2.6632 15.94
202.0790 -2.7119 15.27
202.0868 -2.7163 15.00