TITLE: GCN CIRCULAR NUMBER: 44759 SUBJECT: EP260530a/AT 2026nwl: COLIBRÍ optical observations DATE: 26/06/01 17:34:44 GMT FROM: globus@astro.unam.mx Noémie Globus (UNAM), Damien Dornic (CPPM), 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), , Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We observed the field of the Einstein Probe X-ray transient EP260530a (Wu et al., ATel #17824) with the DDRAGO two-channel wide-field imager mounted on the COLIBRÍ telescope. The observations targeted the optical counterpart candidate, AT 2026nwl, discovered by GOTO (TNS Astronomical Transient Report No. 306622) and associated with EP260530a. We observed from 2026-06-01 04:59 to 06:02 UTC (from (55.93 to 56.97 hours after the GOTO discovery) and obtained 11, 23, 10, 24, and 20 minutes of exposure in the g, r, i, z, and y filters, respectively. 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 detect the optical counterpart previously reported by GOTO (AT 2026nwl). The source is measured at the following preliminary magnitudes: g = 15.22 +/- 0.01, r = 15.33 +/- 0.01, i = 15.41 +/- 0.01, z = 15.52 +/- 0.01, y = 15.58 +/- 0.01. The source is brighter than the magnitude of L = 17.03 at 2026-05-29 21:04 UTC reported by the GOTO team (TNS Astronomical Transient Report 306622), but fainter than the unfiltered magnitude of 14.46 at 2026-05-31 05:24 UTC reported by Sokolovsky et al (TNS Astronomical Transient Report 306654). If the transient has a simple rise and decay, this suggests that it has already reached its maximum. The source shows blue colors, consistent with both a CV and an FBOT. During the 55 minutes of our observations, the source shows a smooth rise from r = 15.35 to r = 15.28 followed by a smooth fall again to r = 15.36. Similar behavior is also seen in the other filters. This variability is reminiscent of that seen in CVs. We encourage spectroscopic observations to determine the redshift of this source and its likely nature. 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/44759. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...