TITLE: GCN CIRCULAR NUMBER: 41958 SUBJECT: GRB 250920B: AbAO optical upper limits DATE: 25/09/23 21:32:13 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), R. Ya. Inasaridze (AbAO) 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 AS-32 0.7m telescope of AbAO. We obtained two series of images in the R band on epochs 2025-09-22 and 2025-09-23 (~2.3 and ~3.3 days since trigger, respectively). 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; Pankov et. al, GCN 41953) is not detected in the stacked images of both epochs. Preliminary upper limits are as follows: Date UTstart Exptime t-T0 Filter OT Err. UL (nxs) (mid, days) (3sigma) 2025-09-22 16:09:15 20x2+97x30 2.32458 R n/d n/d 20.3 2025-09-23 16:04:34 153*30 3.33076 R n/d n/d 20.3 The photometry was calibrated against nearby PS1 DR2 stars (R-mags obtained via Lupton 2005 transformations) and not corrected for the Galactic extinction. Ref. stars PS1 DR2 (Lupton 2005) RA Dec R R_err 187.24325 47.82521 15.134 0.007 187.41433 47.78151 14.974 0.008 View this GCN Circular online at https://gcn.nasa.gov/circulars/41958. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...