{
  "submitter": "freddalvarez@astro.unam.mx",
  "subject": "GRB 260421B: SVOM/COLIBRÍ (FM-GFT) optical detection of a candidate counterpart",
  "createdOn": 1776749286522,
  "submittedHow": "web",
  "eventId": "GRB 260421B",
  "circularId": 44364,
  "body": "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:\n\nWe 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.\n\nThe data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU pipeline.\nThe photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.\n\nWe detect an uncatalogued source consistent with the ECLAIRs localization at: \nRA(J2000) = 12:40:00.48 = 190.00200 degrees\nDec(J2000) = -17:44:24.6 = -17.74015 degrees\nwith an uncertainty of 0.5 arcsec.\n\nThe 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.\n\nSubsequent imaging in the g/r/i/z/y filters show detections in all bands, indicating that the redshift is less than 3.5.\n\nWe encourage spectroscopic observations of the source. Further observations are planned.\nWe 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.\nCOLIBRÍ 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.\n",
  "format": "text/plain"
}