TITLE: GCN CIRCULAR NUMBER: 40083 SUBJECT: GRB 250404A / EP250404a: Leavitt Observatory optical counterpart detection DATE: 25/04/05 22:56:51 GMT FROM: leavittob@gmail.com E. Pavoni, L. Moretti (Leavitt Observatory, Italy) Members of: GRB/UAI - Gamma Ray Burst Section of Unione Astrofili Italiani ATA - Associazione Tuscolana di Astronomia In a large collaboration with: M.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy), K. Kalinowski (Aarhus University, Department of Physics and Astronomy), report: We observed the field of EP250404a detected by Einstein Probe (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB 250404A (Fermi team, GCN 40050), with our RC telescope (D=250 mm, F/D=8) of Leavitt Observatory, Manciano, Italy. Median moon illumination of 47% was present. The observations started approximately 5.1 hours after the trigger time, stacking a series of R-Cousins filtered exposures, for a total integration time of approximately 88 minutes. All images were processed using the astropy package (Astropy Collaboration et al., 2022ApJ...935..167A). We detected the optical counterpart (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Zhu et al., GCN 40061; Konno et al. GCN 40063; Odeh et al., GCN 40064; Lipunov et al., GCN 40066; He et al., GCN 40069; Perez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al., GCN 40073; Taguchi et al., GCN 40074; Ruocco, GCN 40076; Martin-Carrillo et al., GCN 40080) in the stacked image at the following coordinates +/- 2 arcsec: R.A. (J2000) = 08:20:14:59 Dec. (J2000) = +35:31:41:75 with the following photometry: Date UT at mid-exposure: 2025-04-04 20:24:18 UTC Mag (R Cousins) 19.47 Error +/- 0.2 Magnitudes were estimated with the Pan-STARRS cat. and converted using Lupton (2005) equations. No corrected for galactic dust extinction. The message may be cited. Reference: https://leavittobservatory.altervista.org View this GCN Circular online at https://gcn.nasa.gov/circulars/40083. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...