TITLE:   GCN CIRCULAR
NUMBER:  40942
SUBJECT: EP250704a: COLIBRÍ optical counterpart candidate
DATE:    25/07/04 09:58:19 GMT
FROM:    Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>

Benjamin Schneider (LAM), Rosa L. Becerra (U Roma), Sarah Antier (OCA), William H. Lee (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), 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), and Antonio de Ugarte Postigo (LAM):

We imaged the field the EP250704a (Li et al., GCN 40941) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-07-04 08:27 to 08:46 UTC (from 10.8 to 30.0 minutes after the trigger and starting 25 seconds after the arrival of the notice) and obtained 16 minutes of exposure in the i 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 our stacked image, we detect a new source not visible in the PanSTARRS catalogue at:

RA(J2000) = 20:03:29.51 = 300.8729 degrees
Dec(J2000) = +12:01:23.46 = 12.0232 degrees

with an uncertainty of 0.5 arcsec and within the EP/FXT region (Li et al., GCN 40941). We measure a preliminary magnitude of:

i = 20.46 +/- 0.06

We suggest this is the optical counterpart of EP250704a.

Further observations and analysis are ongoing.

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.