TITLE: GCN CIRCULAR NUMBER: 45450 SUBJECT: GRB 260818B: SVOM/COLIBRÍ (FM-GFT) optical counterpart candidate DATE: 26/08/26 16:27:03 GMT FROM: J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr> Jean-Grégoire Ducoin (CPPM), Marion Guelfand (CPPM), Diego López-Cámara (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), 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), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) and Clare Plasse (HKU) report: We continued imaging the field of the SVOM GRB 260818B (Cordier et al., GCN Circ. 45398) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-08-25T07:59:01 to 11:12:53.575 UTC (from 155.80 to 158.95 hours after the trigger) and obtained 144 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 possible optical transient (revealed by image subtraction using Legacy Survey as template) consistent with the ECLAIRs 4.6 arcmin error circle (Cordier et al., GCN Circ. 45398) at: RA(J2000) = 02:59:54.374 = 44.97656 degrees Dec(J2000) = 07:25:28.24 = 7.424511 degrees with an uncertainty of 0.5 arcsec. The preliminary photometry of the transient candidate, measured on the unsubtracted images, is: First epoch, 2026-08-19, 13.20 hr after the trigger: r = 24.24 +/- 0.15 mag z > 23.81 mag (3-sigma upper limit) Second epoch, 2026-08-25, 157.38 h after the trigger: r = 24.60 +/- 0.29 mag z = 22.84 +/- 0.15 mag At the first epoch, the transient candidate is brighter than the reference Legacy Survey source in the r band. At the second epoch, it is also brighter in the z band, showing a clear brightening relative to the first epoch. The source is located 11.1 arcseconds from the X-ray source EPF_J025955.0+072535 reported by Wu et al. (GCN Circ. 45407), and 1.9 arcseconds from the radio source S10 reported by Schroeder et al. (GCN Circ. 45418). We also retrieved an r-band template image from the Hyper Suprime-Cam Legacy Archive. A source is visible at the position of the optical transient candidate and may correspond to its host galaxy. At this stage, we cannot draw firm conclusions regarding the nature of the transient or a possible association with GRB 260818B. Further observations are ongoing. We encourage additional follow-up, particularly at redder wavelengths, to better constrain the nature of this source and assess its possible association with GRB 260818B. 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/45450. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...