TITLE: GCN CIRCULAR NUMBER: 40230 SUBJECT: GRB 250424A: COLIBRÍ Detection of a Bright Optical Counterpart DATE: 25/04/24 10:48:17 GMT FROM: J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Benjamin Schneider (LAM), Ramandeep Gill (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA/IJCLAB), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of Swift/BAT GRB 250424A (Cenko et al., GCN Circ. 40224) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico. We started observing at 2025-04-24T08:39:07 UTC (1.78h after the trigger). The data were coadded with the COLIBRÍ pipeline and analyzed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against SkyMapper DR4. Our photometry is in the AB system and is not corrected for Galactic extinction. In our first 780 seconds of exposure, At the position of the optical counterpart (Francile et al. GCN Circ. 40222; Cenko et al. GCN Circ. 40224; Brivio et al. GCN Circ. 40225; Becerra et al. GCN Circ. 40226; Saccardi et al. GCN Circ. 40228; de Wet et al. GCN Circ. 40229), we detect a source with i = 18.93 +/- 0.05 Further observations and analysis are ongoing. We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir. View this GCN Circular online at https://gcn.nasa.gov/circulars/40230. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...