{
  "circularId": 45654,
  "submittedHow": "web",
  "format": "text/plain",
  "subject": "EP260920a: COLIBRÍ optical upper limits",
  "eventId": "EP260920a",
  "submitter": "Rosa L. Becerra at Instituto de Astronomía,  UNAM <rbecerra@astro.unam.mx>",
  "body": "Damien Dornic (CPPM), Ramandeep Gill (UNAM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Edilberto Aguilar-Ruiz (UNAM), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP),  Leonardo García-García (UNAM), Noémie Globus (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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM),  Alan M. Watson (UNAM) report:\n\nWe imaged the field of the EP260920a (Ni et al., GCN 45649) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-09-20 05:33 to 06:57 (from 5.4 to 6.9  hours after the trigger) and obtained 61 and 56  minutes of exposure in the r and z filters, respectively.\n\nThe data were reduced and coadded with the 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.\n\nIn the stacked images, after performing image subtraction with STDWeb/STDPipe (Karpov 2025), using templates from the DESI Legacy Survey DR10 (Dey et al. 2019), we do not detect the tentative source reported by Li et al. (GCN Circ. 45650; 45653) down to the following 3-sigma limits:\n\nr > 24.0,\nz > 22.9\n\nWe agree with Li et al. (GCN Circ. 45653) that the analysis at the position of the candidate is complicated by the presence of the underlying catalogued point source. In particular, image subtraction at this location is sensitive to differences between the science images and the reference templates, including filter and PSF mismatches, which can produce subtraction residuals. After inspecting both the unsubtracted and difference images, we find no convincing evidence for an additional transient component at this position.\n\nWe also do not detect any uncatalogued source within the FXT region (Ni et al., GCN Circ. 45649) down to the following 5-sigma limits:\n\nr>23.5,\nz>22.3\n\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.\n\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",
  "eventType": [
    "X-ray Transient"
  ],
  "createdOn": 1789891228650
}