TITLE: GCN CIRCULAR NUMBER: 40783 SUBJECT: GRB 250617B: Assy AZT-20 Optical Observations DATE: 25/06/19 14:26:12 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com> N. Pankov (HSE, IKI), V. Kim (FAI), A. Pozanenko (IKI), M. Krugov (FAI) report on behalf of IKI-GRB-FuN: We performed optical observations of the field of GRB 250617B (Page et. al, GCN 40760; The Fermi GBM team, GCN 40763) using the 1.5-meter AZT-20 of the Assy-Turgen Observatory. The observations were conducted starting on 2025-06-18 at 18:02:27 UT, i.e. ~0.87 days since the Swift trigger. The series of 53*60 sec images was taken in the r'-filter. We detect the optical afterglow (Page et. al, GCN 40760, Moskvitin et. al, GCN 40761, Jelinek et. al, GCN 40762; Kumar et. al, GCN 40768; Lipunov et. al, GCN 40769; Grossan et. al, GCN 40771; Ma et. al, GCN 40772; Schneider et. al, GCN 40774; Antier et. al, GCN 40775; Corcoran et. al, GCN 40776; Turpin et. al, GCN 40777; Siegel & Page, GCN 40778). The preliminary photometry is given below: Date UT start t-T0 Exp. Filter OT Err. UL (mid, days) (n*s) (3sigma) 2025-06-18 18:02:27 0.87577 53*60 r' 21.58 0.04 22.5 The photometry has calibrated using nearby stars from PS1 (at the same coordinates as USNO-B1.0 stars given in Moskvitin et. al, GCN 40761) and has not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/40783. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...