TITLE:   GCN CIRCULAR
NUMBER:  41953
SUBJECT: GRB 250920B: CrAO ZTSH optical observations
DATE:    25/09/23 12:38:45 GMT
FROM:    Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>


N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO) report on behalf of GRB-IKI-FuN:

We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the ZTSH telescope of CrAO. We obtained series of images in R,V bands starting on 2025-09-21 at 16:28:47.74 UT, i.e. 1.321 days since Fermi/GBM trigger. The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914; Pankov et. al, GCN 41931; Reguitti et. al, GCN 41932; Moskvitin et. al, GCN 41948) is clearly visible in individual 120 s exposures taken in the R-band. Preliminary photometry of the OT in the R-band is as follows:

Date       UTstart  Exptime    t-T0      Filter OT   Err.    UL
                     (s)     (mid, days)                  (3sigma)
2025-09-21 17:21:58  120      1.35870     R     20.7 0.1     22

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