TITLE: GCN CIRCULAR NUMBER: 39658 SUBJECT: GRB 250309B / AT2025dws: further SAO RAS optical observations DATE: 25/03/10 23:50:28 GMT FROM: Alexander Moskvitin at SAO RAS mosk@sao.ru

A. S. Moskvitin, O. I. Spiridonova (SAO RAS), A. S. Pozanenko (IKI), A. Ghosh, S. Razzaque (CAPP, University of Johannesburg), Yu. V. Sotnikova (SAO RAS) report on behalf of a larger collaboration.

We observed the field of GRB 250309B / AT2025dws (Preis and Greiner, GCN 39629; Fermi GBM team, GCN 39635; McDermott et al., GCN 39642; Wang et al., GCN 39648; Page and Evans, GCN 39649; Kozyrev et al., GCN 39652; Frederiks et al., GCN 39655; Scotton, GCN 39657) with Zeiss-1000 1-m telescope of SAO RAS, equipped with the CCD-photometer. We obtained 19 x 300 sec. images in Rc band on March 10, 21:37:13 -- 23:23:40 UT (t_mid - T0 = 1.6194 days).

The optical candidate ZTF25aaitvjt / AT2025dws (Stein et al., GCN 39639; Pérez-Fournon et al., GCN 39643; Stein and Ahumada, GCN 39644; Moskvitin et al., GCN 39645; Perley et al., GCN 39646; Malesani et al., GCN 39647; Ducoin et al., GCN 39650; Shin et al, GCN 39654) is clearly detected in the stacked image with the brightness of R = 20.65 +/- 0.13.

We do not performed host galaxy subtraction. The photometry is based on nearby stars from the USNO-B1.0 catalog (R2 magnitudes) and has not been corrected for the Galactic extinction.

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