TITLE: GCN CIRCULAR NUMBER: 44216 SUBJECT: EP260324A / AT 2026hir: COLIBRÍ further optical observations DATE: 26/04/05 05:38:58 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Massimiliano Lincetto (CPPM), Rosa L. Becerra (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), 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 AT2026hir/ZTF26aapviim, the optical counterpart (Anumarlapudi et al., GCN Circ. 44120, Stein et al., GCN Circ. 44194, Globus et al., GCN Circ. 44196) of EP260324a (Wu et al., Atel #17728), with the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-05 03:10 to 03:26 UTC (from 11.95 to 11.96 days after the discovery time reported in the TNS record and from 11.84 to 11.85 days after the EP trigger) and obtained 5 minutes of exposure in each of the g, r, i filters, and 15 minutes of simultaneous exposure in the z filter. The data were reduced and coadded with the 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. In the stacked image, we detect the optical counterpart. The preliminary PSF-fitting magnitudes after performing image subtraction using PanSTARRS DR1 images as a template are: r = 18.01 +/- 0.01, z = 18.54 +/- 0.02. Compared to our previous observations from 2026-04-03 (Globus et al., GCN Circ. 44196), we see an apparently achromatic fading of: delta r = 0.28 +/- 0.01, delta z = 0.28 +/- 0.03. Further observations are planned. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44216. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...