TITLE: GCN CIRCULAR NUMBER: 39687 SUBJECT: GRB 250309B/AT 2025dws: Daochen/RC-10 optical counterpart observation DATE: 25/03/12 17:01:44 GMT FROM: sqjiang at NAOC sqjiang@bao.ac.cn

S.Q. Jiang (NAOC), S.H. Wang (THU), J. An, X. Liu, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu (HUST), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:

We observed the optical counterpart AT 2025dws (Stein et al., GCN 39639, 39644; Pérez-Fournon et al., GCN 39643; Moskvitin et al., GCN 39645, 39658; Perley et al., GCN 39646; Malesani et al., GCN 39647; Page and Evans, GCN 39649; Ducoin et al., GCN 39650; Shin et al., GCN 39654; Wang et al., GCN 39661; Guiffreda et al., GCN 39670) of GRB 250309B (GBM team, GCN 39635; Preis and Greiner, GCN 39629; McDermott et al., GCN 39642; Wang et al., GCN 39648; Kozyrev et al., GCN 39652; Frederiks et al., GCN 39655), using the 0.33 m diameter telescope RC-10 located at Daochen, Sichuan, China. Observations started at 18:53:25.836 on 2025-03-09, i.e., 11.247 hrs after the Fermi/GBM trigger time, a series of 600 s frames in sloan r-band were obtained.

The optical transient was detected in our stacked image with mag_r= 19.7 +/- 0.2 at a median time of 12.260 hrs after the trigger time, calibrated with the nearby Pan-STARRS field and not corrected for Galactic extinction.

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