TITLE: GCN CIRCULAR NUMBER: 40072 SUBJECT: EP250404A / GRB250404A: Ondrejov D50 optical afterglow detection DATE: 25/04/05 03:49:34 GMT FROM: Martin Jelinek at Astro.Inst-AVCR,Ondrejov martin.jelinek@asu.cas.cz

M. Jelinek, J. Strobl, F. Novotny, R. Hudec (ASU CAS Ondrejov) report:We observed the field of EP250404a detected by EP (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB 250404A (Fermi team, GCN 40050; Mukherjee et al., GCN 40067), using the D50 telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. Our observations started at 15:50 UT on 2025-04-04, approximately 5.5 hours after the trigger. We performed a series of 120s exposures in the SDSS i' filter. The optical counterpart, previously reported by several teams (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Konno et al., GCN 40063; Odeh et al., GCN 40064; He et al., GCN 40069; Pérez-Fournon et al., GCN 40071), is detected in our combined image of 25x120s exposures. The PanSTARRS-calibrated magnitude of the source is i' = 19.5 +/- 0.1, not corrected for Galactic extinction.

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