TITLE: GCN CIRCULAR NUMBER: 44790 SUBJECT: EP260602a: COLIBRÍ optical observations DATE: 26/06/03 06:45:07 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Marion Guelfand (CPPM), 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), 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 imaged the field of EP260602a (Li et al., GCN Circ. 44770) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-03 03:38 to 05:17 UTC (from 19.45 to 21.10 hours after the trigger) and obtained 15, 30, 29, and 74 minutes of exposure in the g, r, i, and z 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 a faint source at a position consistent with that of the counterpart reported by Xu et al. (GCN Circ. 44781). This source is not seen in Pan-STARRS DR2 or Legacy Survey DR10 (Dey et al. 2019) images. The preliminary magnitudes of the source are, after subtracting the PS images to remove the 20-mag source 3.5 arcsec to the south, are: g = 23.27 +/- 0.40 r = 23.26 +/- 0.21 i = 23.28 +/- 0.24 z = 22.98 +/- 0.39 The colors in our photometry are moderately red and do not suggest a high-redshift counterpart. Xu et al. report a very red color of VT_B - VT_R ≈ 2.0 +/- 0.3. Comparing griz photometry to VT photometry is not straightforward, but approximating VT_B ≈ g + r and VT_R ≈ i + z, we estimate much less extreme colors of VT_B - VT_R ≈ 0.13 +/- 0.32. One possible explanation for this difference is that Xu et al. observed a high-redshift or highly-reddened afterglow which has now faded, and we are observing the host or an intervening galaxy. However, the lack of detection of a corresponding source in both Pan-STARRS and Legacy Survey images argues against this. We currently do not have an explanation for this difference. 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/44790. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...