TITLE:   GCN CIRCULAR
NUMBER:  41981
SUBJECT: GRB 250924A: SAO RAS optical observations
DATE:    25/09/25 10:53:27 GMT
FROM:    Alexander Moskvitin at SAO RAS <mosk@sao.ru>

A. Moskvitin (SAO), V. Goranskij (SAI MSU), 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 250924A (Siegel et. al, GCN 41897;
Osborne et. al, GCN 41973; Campana et. al, GCN 41974) with two SAO RAS
telescopes, 1-m Zeiss-1000 equipped with the CCD-photometer on
September 24/25, 23:41:32--00:27:16, and 6-m BTA equipped 
with the focal reducer SCORPIO-2 (Afanasiev & Moiseev,  2011, BaltA, 
20, 363) on September 25, 01:42:55--01:58:24 UT.

The optical transient (Siegel et. al, GCN 41959; Becerra et. al, 
GCN 41960; Schneider et. al, GCN 41963; Freeberg et. al, GCN 41968;
Pankov et. al, GCN 41969; Zheng and Filippenko, GCN 41970; 
Siegel, GCN 41975; Ma et al., GCN 41976; Cotter et al., GCN 41980; 
and also observed in NIR, Mo et. al, GCN 41972) is clearly detected 
in our stacked frames. Preliminary results are as following.

Date       UTstart   Exptime   t-T0       Filter OT    Err.    UL  Tel.
                     s         (mid, days)                (3sigma)
2025.09.24 23:41:32  2520   0.65689       Rc  23.18 +/- 0.24  23.5 1-m
2025.09.25 01:42:55  600    0.73068       Rc  23.07 +/- 0.10  24.3 6-m

The photometry calibrated against nearby USNO-B1.0 stars (R2 magnitudes)
and not corrected for the Galactic extinction.