TITLE: GCN CIRCULAR NUMBER: 39645 SUBJECT: GRB 250309B / AT2025dws: SAO RAS optical observations DATE: 25/03/10 02:45:01 GMT FROM: Moskvitin Alexander 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 partially covered the error circle of the GRB 250309B (Preis and Greiner, GCN 39629; Fermi GBM team, GCN 39635; McDermott et al., GCN 39642) with 1-m telescope of SAO RAS, Zeiss-1000, equipped with the CCD-photometer.
The fields of ZTF25aaitvjt / AT2025dws (Stein et al., GCN 39639), XRT sources #1 and #3 (Evans et al., GCN 39633) were observed in Rc band on March 9/10 night. The AT2025dws source does not fit into the error circle of ground-based GBM localization (Fermi GBM group, GCN 39635), but falls into the BALROG localization field (Preis et al., GCN 39629).
We obtained two epochs of observations for ZTF25aaitvjt / AT2025dws field. The 1st: on March 9, 21:58:17 -- March 9, 22:29:28 UT, t_mid - T0 = 14.5893 h = 0.60789 days (5 x 300 sec. exposure); the 2nd: on March 9, 23:52:03 -- March 10, 00:47:22 UT, t_mid - T0 = 16.6865 h = 0.69527 days (10 x 300 sec. exposure).
We clearly detected ZTF25aaitvjt / AT2025dws (Stein et al., GCN 39639; Pérez-Fournon et al., GCN 39643; Stein and Ahumada, GCN 39644) in the stacked images. Preliminary photometry is the following.
Date UT start t-T0 Exptime Filter OT Err UL (mid, days) (s) (3sigma) 2025-03-09 21:58:17 0.60789 5 x 300 Rc 19.52 +/- 0.04 22.0 2025-03-09 23:52:03 0.69527 10 x 300 Rc 19.66 +/- 0.02 22.6
The above photometry includes the OT and a possible host galaxy. The photometry is based on nearby stars from the USNO-B1.0 catalog (R2 magnitudes) and has not been corrected for the Galactic extinction. USNO-B1.0 reference stars RA Dec R2 14:03:06.24 -08:27:38.9 15.67 14:03:08.07 -08:26:47.1 17.30
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