TITLE: GCN CIRCULAR NUMBER: 39730 SUBJECT: GRB 250226A: Mondy AZT-33IK and Koshka Zeiss-1000 Optical Upper Limits DATE: 25/03/15 07:21:33 GMT FROM: Nicolai Pankov at HSE, IKI RAS colinsergesen@gmail.com

N. Pankov (HSE, IKI), E. Klunko (ISTP), A. Novichonok (KIAM), I. Nikolenko (INASAN), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of GRB 250226A (The Fermi GBM team, GCN 39479; V.Lipunov et. al, GCN 39481; Jiang et. al, GCN 39482; An et. al, GCN 39486; Zhu et. al, GCN 39487; Magnani et. al, GCN 39488; Li et. al, GCN 39489; Li et. al, GCN 39492; Aryan et. al, GCN 39509; Zou et. al, GCN 39511; Jiang et. al, GCN 39513; Li et. al, GCN 39514; Junjie-Jin et. al, GCN 39515; Thakur et. al, GCN 39518; Poidevin et. al, GCN 39524; Pathak et. al, GCN 39530; Pankov et. al, GCN 39539; Pankov et. al, GCN 39540; Svinkin et. al, GCN 39542; Tanasan et. al, GCN 39554) in the Rb filter with 1-meter Zeiss-1000 telescope of the Koshka Observatory (INASAN), and in the R-filter with 1.5-meter AZT-33IK telescope of Sayan Solar Observatory (Mondy). The observations began on 2025-02-28 23:27:51 UT, i.e. ~2.75 days since trigger at Koshka. We do not detect the optical counterpart in the stacked images from both telescopes. The preliminary upper limits are as follows:

Date UT start t-T0 Exp. Filter UL(3sigma) (mid, days) (s) 2025-02-28 23:27:51 2.75236 47*180 Rb 22.1 2025-03-01 20:06:19 3.60304 57*120 R 23.8

Ref. stars USNO-B1.0 RA Dec R2 14:57:12.82 +20:57:33.04 15.73 14:57:12.66 +21:00:25.03 15.74 14:56:52.84 +21:00:58.82 14.86

The photometry is based on nearby stars from the USNO-B1.0 catalog (see above) and has not been corrected for the Galactic extinction.

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