TITLE: GCN CIRCULAR NUMBER: 40762 SUBJECT: GRB 250617B: Ondrejov D50 optical afterglow detection DATE: 25/06/17 22:55:14 GMT FROM: Martin Jelinek at Astro.Inst-AVCR,Ondrejov martin.jelinek@asu.cas.cz

M. Jelinek, J. Strobl, F. Novotny, A. Malenakova and R. Hudec (ASU CAS Ondrejov) report: We observed the position of the Swift-detected GRB 250617B (Page et al. GCN 40760) with the D50 telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. Our observation started at 21:29:35 UT, i.e. 27.8 min after the initial trigger and was performed without filter. We detect the optical afterglow (Page et al. GCN 40760) in a combined image with exp. mean time ~37.3 min after trigger (exposed 21:29:35-21:39:35 UT). The AB magnitude of the object at this frame is R = 18.64 +/- 0.27. We can confirm a decaying nature of the object with a temporal decay index of alpha = 0.85 ± 0.13.

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