TITLE: GCN CIRCULAR NUMBER: 42083 SUBJECT: GRB 251001B: AbAO optical upper limit DATE: 25/10/03 09:51:25 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO) report on behalf of IKI-GRB-FuN: We performed optical observations of the field of GRB 251001B detected by Fermi/GBM (The Fermi GBM team et. al, GCN 42038), and Swift/BAT (Beardmore et. al, GCN 42039) with the AS-32 0.7m telescope of the Abastumani Observatory (AbAO). The R-filter observations began on 2025-10-01 22:37 UT, i.e. ~0.35 days since trigger and consisted of 55x30 s exposures. The optical source at z = 2.162 (Sanchez-Ramirez et. al, GCN 42053) found by GOTO (Gompertz et. al, GCN 42040) and also observed by (An et. al, GCN 42041; Strausbaugh & Cucchiara, GCN 42043; O'Neill et. al, GCN 42046; de Ugarte Postigo et. al, GCN 42050; Ma et. al, GCN 42051; Sanchez-Ramirez et. al, GCN 42053; Schneider et. al, GCN 42054; Breeveld et. al, GCN 42055) is not detected in the co-add image of 51x30 s. Preliminary photometry and observation details are presented below: Date UT start t-T0 Exp. Filter OT Err. UL (mid, days) (s) (3sigma) 2025-10-01 22:37:48 0.35641 51x30 R n/d n/d 19.9 The photometry was calibrated using nearby stars from USNO-B1.0 (R2 magnitudes) and not corrected for the Galactic extinction. Ref. stars USNO-B1.0 RA Dec R2 42.63445 -23.02161 14.13 42.77496 -23.05166 13.47 View this GCN Circular online at https://gcn.nasa.gov/circulars/42083. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...