TITLE: GCN CIRCULAR NUMBER: 41248 SUBJECT: EP250806a: COLIBRÍ optical upper limit DATE: 25/08/06 14:02:40 GMT FROM: F. Fortin at IRAP ffortin.sci.edu@gmail.com

Francis Fortin (IRAP), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):

We imaged the field of the EP X-ray transient EP250806a (Yang et al., GCN Circ. 41246) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-06T10:27:49 to 11:10:24 UTC (from 1.1 to 1.9 hours after the trigger) and obtained 32 minutes of exposure in the i-band filter.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 do not detect any new source at the FXT source position (Yang et al., GCN Circ. 41246) down to the following 3-sigma limit:

i > 23.38

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

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.

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