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
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