TITLE: GCN CIRCULAR NUMBER: 41428 SUBJECT: GRB 250818B: SAO RAS optical observations DATE: 25/08/19 08:13:33 GMT FROM: Alexander Moskvitin at SAO RAS mosk@sao.ru

A. Moskvitin, O. Spiridonova, V. Vlasyuk, Yu. Sotnikova (SAO RAS), Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) report on behalf of a larger collaboration.

We observed the field of the SVOM short GRB 250818B (Wang et al., GCN 41405; also detected by Swift/XRT, Ferro et al., GCN 41407 and EP-FXT, Li et al., GCN 41424) with the 1-m SAO RAS telescope Zeiss-1000 equipped with CCD-photometer. We obtained 13 x 300 sec. images in Rc band on August 18, 23:15:02 -- August 19, 00:33:37 UT (t_mid - T0 = 20.4196 hours = 0.85082 days).

The OT (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Fong et al., GCN 41419; Bendtsen et al., GCN 41426) is clearly detected in the stacked image with the brightness of R = 20.88 +/- 0.04 (R_lim = 23.0).

This preliminary photometry is based on nearby Pan-STARRS objects (magnitudes were converted with the Lupton 2005 equations) and not corrected for Galactic extinction.

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