TITLE: GCN CIRCULAR NUMBER: 44274 SUBJECT: GRB 260411B: COLIBRÍ optical observations DATE: 26/04/12 05:49:16 GMT FROM: freddalvarez@astro.unam.mx Fredd Sánchez Álvarez (UNAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Noémie Globus (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), 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), Benjamin Schneider (LAM) report: We imaged the possible optical counterpart (O’Neill et al., GCN Circ. 44265) of the Fermi GRB 260411B (Fermi GBM Team, GCN Circ. 44262) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-12 02:56 to 2026-04-12 04:43 (from 18.86 to 20.68 hours after the trigger) and obtained 25/35/24/84 minutes of exposure in the g/r/i/z filters. The data were reduced, coadded, calibrated, and analyzed 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. In the stacked image, we detect the optical counterpart reported by O’Neill et al. (GCN Circ. 44265), Gupta et al. (GCN Circ. 44271), and Jelinek et al. (GCN Circ.44272) and obtain the following preliminary magnitudes: g = 21.44 +/- 0.04 r = 20.69 +/- 0.02 i = 20.35 +/- 0.03 z = 20.10 +/- 0.03 After correcting for Galactic extinction (A_g ≈ 0.92 and A_z ≈ 0.41), the g-z colour is 0.85 +/- 0.05. This and the other colours are consistent with an afterglow with moderate reddening from its host galaxy. Furthermore, the detection in g-band suggests that the redshift of the transient is likely z < 3.5, assuming typical IGM absorption. Comparing our r magnitude to the one reported by DOT (Gupta et al. GCN Circ. 44271), we estimate that the temporal decay index between 12.4 and 19.6 hours is α ≈ -2.3. The transient appears to show an initially rapid decay in the early GOTO observations (O’Neill et al. GCN Circ. 44265) at about 0.9 hours, followed by a shallower phase (α ≈ −0.8) between 0.9 and 12.4 hours, and a subsequent steepening at later times. In the Pan-STARRS DR1 images, there is an extended source at about 2” from the position of the transient with preliminary magnitudes of: g = 24.12 +/- 0.23 r = 24.06 +/- 0.31 i = 24.64 +/- 0.53 z = 23.44 +/- 0.38 After correcting for Galactic extinction, we estimate a probability of chance coincidence of about 2%, which suggests that this source is the host galaxy of GRB 260411B. We encourage spectroscopic observations of the source. 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. 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/44274. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...