TITLE: GCN CIRCULAR NUMBER: 41255 SUBJECT: EP250806a: Earlier Detection of the VT Candidate by COLIBRÍ and LS DATE: 25/08/07 03:11:09 GMT FROM: Alan Watson at UNAM alan@astro.unam.mx

Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Fredd Sánchez Álvarez (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Camila Angulo (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (AUS), Sarah Antier (OCA), 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), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

In Fortin et al. (GCN Circ. 41248), we reported no optical counterpart of the EP X-ray transient EP250806a (Yang et al., GCN Circ. 41246) in a 32-minute exposure in i taken with COLIBRÍ. We here discuss our full 60-minute exposure in i taken from 2025-08-06 10:27 to 11:48 UTC (from 1.1 to 2.5 hours after the trigger).

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.

At the position of the VT optical candidate reported by Xin et al. (GCN Circ. 41254), we detect a source with

i = 22.47 +/- 0.11

This magnitude is similar to the magnitude of VT_R = 22.6 +/- 0.2 about 8 hours after the trigger reported by Xin et al.

Furthermore, the source is visible in the Legacy Survey DR10 (Dey et al. 2019) catalog with

i = 22.0 +/- 0.07

In both our image and the LS image, the source appears to be resolved.

Given the detection in the LS image, the lack of fading between our observations and the VT observations, and the resolved appearance, we suggest that this is not the optical counterpart of the EP transient.

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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