TITLE:   GCN CIRCULAR
NUMBER:  41948
SUBJECT: GRB 250920B: SAO RAS optical observations 
DATE:    25/09/22 22:46:39 GMT
FROM:    Alexander Moskvitin at SAO RAS <mosk@sao.ru>

A. Moskvitin, O. Spiridonova (SAO), N. Pankov (HSE), A. Pozanenko 
(IKI) report on behalf of GRB follow-up collaboration and IKI-GRB-FuN.

We observed the field of the GRB 250920B (Fermi GBM Team, GCN 41897; 
Gupta et al., GCN 41898; Page et al., GCN 41909; Evans et al., 
GCN 41910; Sonawane and Meegan, GCN 41919; Sakamoto et al., GCN 41923;
Cheung et al., GCN 41937; Svinkin et al., GCN 41942) with the 1-m 
SAO RAS telescope Zeiss-1000 equipped with CCD-photometer. 
We obtained 10 x 300 sec. images Rc band on September 22, 
16:22:43--17:17:34 (t_mid - T0 = 2.33586 days).

The optical transient (Gupta et al., GCN 41898; Wu et al., GCN 41908;
Hall et al., GCN 41914; Kuin and Gupta, GCN 41915; Reguitti et al., 
GCN 41932) is clearly detected in the stacked frame 
with the brightness of R = 21.7 +/- 0.1.

Date        UT start  t-T0     Exp.  Filter  OT     Err. UL (3sigma)
                      (mid, d) (s)      
2025.09.22  16:22:43   2.33586 10 x 300 Rc  21.7 +/- 0.1  23.2

Preliminary photometry is based on the nearby stars from USNO-B1
(R2 magnitudes) and not corrected for the Galactic extinction.