TITLE: GCN CIRCULAR NUMBER: 41931 SUBJECT: GRB 250920B: Kitab-ISON optical upper limit DATE: 25/09/21 17:38:23 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Novichonok (Petrozavodsk State University, KIAM) 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 RC-36 telescope of Kitab-ISON. The series of unfiltered 325x15 sec exposures was obtained starting on 2025-09-20 at 14:31 UT, i.e. ~0.26 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) is not visible in our stacked image of 298x15 sec. Preliminary results of our observations provided below: Date UTstart Exptime t-T0 Filter OT Err. UL (nxs) (mid, days) (3sigma) 2025-09-20 14:31:30 298x15 0.26550 Clear n/d n/d 15.4 The photometry was calibrated against nearby USNO-B1.0 stars (R2 mags) and not corrected for the Galactic extinction. Ref. stars USNO-B1.0 RA Dec R2 187.73536 47.71878 13.75 187.52490 47.74643 13.74 View this GCN Circular online at https://gcn.nasa.gov/circulars/41931. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...