TITLE: GCN CIRCULAR NUMBER: 41074 SUBJECT: EP250711a: COLIBRÍ refined photometry and optical confirmation of the candidate DATE: 25/07/12 13:03:56 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma rosa.becerra@roma2.infn.it

Rosa L. Becerra (U Roma), Jean-Luc Atteia (IRAP), Kin Ocelotl López (UNAM), Stéphane Basa (UAR Pytheas), Alan M. Watson (UNAM), William H. Lee (UNAM), Fredd Alvarez (UNAM), Camila Angulo (UNAM), 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), 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 continued the follow-up campaign of EP250711a (Wang et al., GCN Circ. 41070) using the DDRAGO wide-field imager on the COLIBRÍ telescope. Observations were conducted on 2025-07-12 from 03:40 to 05:46 UTC (10.5 to 12.6 hours after the trigger), obtaining 96 minutes of exposure in the i band. Additional observations were carried out from 2025-07-12 09:46 to 11:21 UTC (16.6 to 18.2 hours after the trigger), with a total of 64 minutes of exposure in the r band.

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.

Due to the significantly larger number of images in the stacks than in López et al. (GCN Circ. 41069), it is now possible to confirm the candidate reported by Fu et al. (GCN Circ. 41071). We measure preliminary magnitudes of:

r = 22.02 +/- 0.17

i = 21.13 +/- 0.11

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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