TITLE: GCN CIRCULAR NUMBER: 40068 SUBJECT: GRB 250403A: Montarrenti Observatory optical detection DATE: 25/04/04 21:35:28 GMT FROM: Simone Leonini at Montarrenti Observatory (Siena, Italy) <s.leonini@iol.it> S. Leonini, M. Conti, P. Rosi, L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy, part of UAI/SSV-GRB section), M.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy) and K. Kalinowski (Aarhus University, Department of Physics and Astronomy) report: we observed the field of GRB 250403A detected by Fermi/GBM (Fermi team, GCN 40025), SVOM/ECLAIRs (Julakanti et al., GCN 40026) and SWIFT (Dichiara et al., GCN 40037; Shilling et al., GCN 40048) with the automated and remoted 0.53m Ritchey-Chretien telescope at Montarrenti Observatory (Siena, Italy, IAU code C88). The observations were started at 2025-04-03 21:56:50 UT (approximately 7 hours after burst) stacking a set of Rc-band CCD images. The elevation of the field decreased from 19 degrees above the horizon. The optical afterglow (Starling et al., GCN 40028; Wu et al., GCN 40029; Du et al., GCN 40031; Li et al., GCN 40032; Julakanti et al., GCN 40034; Ghosh et al, GCN 40039; Jiang et al., GCN 40041; Perez-Fournon et al., GCN 40043; Brivio et al., GCN 40044; Xin et al, GCN 40045 and Zheng et al., GCN 40065) was barely detected (S/N=2.4) at the following position: RA (J2000.0) 12h 58m 27.72s +/-0.15 Decl. (J2000.0) -24° 22' 46.5" +/-0.19 Preliminary photometry was obtained using nearby PanSTARRS stars as follows: Observation Mid-Time T-T0 (hr) Exposure Filter Mag. Err. UL (3-sigma) 2025-04-03 21:57:10 UT 6.68 94x40s Rc 20.10 S/N =2.4 >19.90 Magnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. No correction for galactic dust extinction was applied. View this GCN Circular online at https://gcn.nasa.gov/circulars/40068. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...