TITLE: GCN CIRCULAR NUMBER: 44797 SUBJECT: GRB 260603A: SVOM/COLIBRÍ (FM-GFT) photometric redshift z = 3.57 DATE: 26/06/03 10:00:53 GMT FROM: Marion G <marion.guelfand@gmail.com> Marion Guelfand (CPPM), Alan M. Watson (UNAM), Jean-Grégoire Ducoin (CPPM), Benjamin Schneider (LAM), 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), Rosa L. Becerra (UNAM), 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), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Antonio de Ugarte Postigo (LAM), Ruizhi Li (YNAO), and Ziming Wang (BNU) report: We continued to image the field of the SVOM/ECLAIRs GRB 260603A (Zhao et al., GCN Circ. 44787) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-03 06:27 to 07:24 UTC (from 0.83 to 1.79 hours after the trigger) and obtained 16, 16, 16, 16, and 32 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. The candidate counterpart reported by Guelfand et al. (GCN Circ. 44791) is detected with preliminary magnitudes of: g = 21.97 +/- 0.10 r = 21.23 +/- 0.05 i = 20.96 +/- 0.05 z = 20.87 +/- 0.09 y = 20.91 +/- 0.18 The counterpart has continued to fade. After correcting for the Galactic foreground extinction corresponding to a reddening of E(B-V) = 0.019 mag (Schlafly & Finkbeiner 2011) and fitting a power-law model to the grizy-bands using no dust, SMC, LMC and MW extinction curves, we derive a photometric redshift of z = 3.57 +- 0.42 (1-sigma c.l.). We note the presence of a galaxy about 3.5 arcsec to the north of the counterpart. This galaxy is seen in both Pan-STARRS DR2 and LS DR10 images and has g ≈ 22.11 +/- 0.04, r ≈ 21.33 +/- 0.03, and z ≈ 20.93 +/- 0.04 in the LS DR10. However, the photometric redshift in the LS DR9 is 0.343 +/- 0.088, which is not compatible with the derived photometric redshift of the counterpart, and so this galaxy is unlikely to be the host. On the other hand, a deep g-band image from the Hyper Suprime-Cam Legacy Archive (HSCLA, Tanaka et al. 2021) reveals an underlying object at a position consistent with the optical counterpart, suggesting a possible host galaxy. We encourage spectroscopic observations of the counterpart. 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/44797. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...