TITLE: GCN CIRCULAR NUMBER: 39865 SUBJECT: GRB 250321B: ISON-Kitab RC-36 Optical Upper Limit DATE: 25/03/24 17:11:05 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), A. Schmalz (KIAM), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN: We performed optical observations of the field of GRB 250321B (Schanne et. al, GCN 39818; Capalbi et. al, GCN 39856) in the Clear light with the 0.36-meter RC-36 telescope of the ISON-Kitab observatory. The observations started on (UT) 2025-03-21 17:31:49, i.e. about 0.59 days since trigger. We do not detect an optical afterglow candidate in the stacked image. The preliminary photometry is as follows: Date UT start t-T0 Exp. Filter OT Err UL(3sigma) (mid, days) (s) 2025-03-21 17:31:49 0.59330 38*60 Clear n/d n/d 17.0 Ref. stars RA Dec r 169.5809 +39.4176 13.063 169.7213 +39.4809 13.341 The photometry is based on nearby stars of SDSS-DR12 (r-magnitudes) and has not been corrected for the Galactic extinction. Our result is consistent with non-detections of an optical (Ducoin et. al, GCN 39840) and an X-Ray (Capalbi et. al, GCN 39856) counterpart. View this GCN Circular online at https://gcn.nasa.gov/circulars/39865. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...