GCN Circular 44364
Subject
GRB 260421B: SVOM/COLIBRÍ (FM-GFT) optical detection of a candidate counterpart
Event
Date
2026-04-21T05:28:06Z (3 days ago)
From
freddalvarez@astro.unam.mx
Via
Web form
Fredd Sánchez Álvarez (UNAM), Nikos Mandarakas (LAM), Alan M. Watson (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), Rosa L. Becerra (UNAM), 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), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), W.L. Zhang (PMO), and Z. Y. Liu (GZNU) report:
We imaged the field of the SVOM/ECLAIRs and Fermin/GBM GRB 260421B (Zhao et al., GCN Circ. 44362; Fermi GBM Team, GCN Circ. 44360) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-21 04:15:13 to 04:30:07 (from 1.9 to 16.8 minutes after the trigger) and obtained 430 seconds of simultaneous exposure in the r/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.
We detect an uncatalogued source consistent with the ECLAIRs localization at:
RA(J2000) = 12:40:00.48 = 190.00200 degrees
Dec(J2000) = -17:44:24.6 = -17.74015 degrees
with an uncertainty of 0.5 arcsec.
The source was detected in both r and z and was observed to fade from r ≈ 14.5 to r ≈ 18.3 during our observations. The color of the source was r - z ≈ 0.45 +/- 0.02, which is consistent with the expectations for an afterglow. We suggest this is the optical counterpart of the GRB.
Subsequent imaging in the g/r/i/z/y filters show detections in all bands, indicating that the redshift is less than 3.5.
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.