TITLE: GCN CIRCULAR NUMBER: 44131 SUBJECT: EP260327a: COLIBRÍ optical upper limits DATE: 26/03/27 17:16:15 GMT FROM: Marion G <marion.guelfand@gmail.com> Marion Guelfand (CPPM), Enrique Moreno Méndez (UNAM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Benjamin Schneider (LAM), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the X-ray transient EP260327a (Dai et al., GCN Circ. 44126) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-27 06:03:18 to 08:10:24 UTC (from 8.6 to 135 minutes after the trigger) and obtained 5760 seconds of simultaneous exposure in the r/z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline and the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In the stacked image, and after subtracting template images from the Subaru template images from Hyper Suprime-Cam Strategic Program (Aihara et al. 2021, doi:10.1093/pasj/psab122), we do not detect any new source at the FXT refined position (Dai et al., GCN Circ. 44126) down to the following 5-sigma limits: r > 24.4, z > 23.3. We do not find any optical transient at the position of the Gemini candidate (Rob Eyles-Ferris et al GCN Circ. 44129), although our observations started approximately three hours earlier. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/44131. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...