TITLE: GCN CIRCULAR NUMBER: 44745 SUBJECT: AT 2026nuz: COLIBRÍ optical observations DATE: 26/05/30 16:56:24 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the AT 2026nuz (Habeeb et al., GCN Circ. 44737) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-30 08:34 to 11:17 UTC (from 30.4 to 32.4 hours after the trigger for GRB 260529A; Fermi GBM team, GCN 44733) and obtained 30, 30, 62, and 122 minutes of exposure in the g, r, i, and z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. We detected the optical transient AT 2026nuz reported by Habeeb et al. (GCN Circ. 44737) at preliminary magnitudes of: g = 18.29 +/- 0.01 r = 18.32 +/- 0.01 i = 18.46 +/- 0.01 z = 18.51 +/- 0.01 The blue colors suggest that this is not the afterglow of GRB 260529A but could be CV, as suggested by Habeeb et al., or possibly an FBOT. That said, in addition to the reasons given by Habeeb et al., the lack of an obvious host in Legacy Survey imaging (Dey et al. 2019) argues against this being an FBOT. Nevertheless, we encourage spectroscopic observation to determine the nature of this source. 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/44745. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...