TITLE: GCN CIRCULAR NUMBER: 41342 SUBJECT: EP250813a: COLIBRÍ detection of the VT candidate and evidence of flattening DATE: 25/08/13 13:44:03 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma rosa.becerra@roma2.infn.it
Jean-Grégoire Ducoin (CPPM), 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 (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (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):
We imaged the field of the EP250812a (Zhang et al., GCN Circ. 41327, 41339) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-13 08:00 to 10:21 UTC (from 18.8 to 21.2 hours after the trigger) and obtained 96 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 the stacked image, we marginally detect the VT candidate (Xin et al., GCN Circ. 41330), which is also consistent with the XRT Source 1 position (Evans et al., GCN Circ. 41340), at:
i = 22.5 +/- 0.3
This value is similar to the second observation reported by VT (Xin et al., GCN Circ. 41330) at T + 14.34 h, but brighter than in the Legacy Survey DR10 catalog (Dey et al. 2019):
i = 23.5 +/- 0.1
Therefore, we suggest that the candidate is indeed the afterglow of EP250812a and that it showed evidence of a flattening in its evolution at the time of our observations.
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.
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