TITLE:   GCN CIRCULAR
NUMBER:  44220
SUBJECT: EP260405a / EP-WXT 11900672904: COLIBRÍ optical upper limits
DATE:    26/04/05 16:04:19 GMT
FROM:    Massimiliano Lincetto at Aix Marseille Univ, CNRS/IN2P3, CPPM <lincetto@cppm.in2p3.fr>

Massimiliano Lincetto (CPPM), Alan M. Watson (UNAM), Jean-Grégoire Ducoin (CPPM), 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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of the EP-WXT transient EP260405.149 using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. Observation started within 30 seconds from the alert reception. We observed on 2026-04-05 from 09:51:17 to 10:30:32 UTC (from 6.25 to 7 hours after the trigger T0) and obtained 1920 seconds of simultaneous exposure in the r and z filters.

The data were reduced and co-added with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the stacked image, we do not detect any new source at the WXT source position down to the following 3-sigma limit:

r > 21.11
z > 20.57

Further observations may be 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.