TITLE: GCN CIRCULAR NUMBER: 40431 SUBJECT: EP250511a: COLIBRÍ optical counterpart candidate DATE: 25/05/11 14:01:49 GMT FROM: Benjamin Schneider at MIT bschn@mit.edu
Benjamin Schneider (LAM), Kin Ocelotl López (UNAM), Rosa L. Becerra (U Roma), Camila Angulo (UNAM), 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), Francis Fortin (IRAP), Noémie Globus (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 of the EP250511a (Lian et al., GCN Circ. 40429) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-05-11 09:25:48 to 10:08:02 UTC (from 1.43 to 2.13 hours after the trigger, and 2 minutes after the notice) and obtained 32 minutes of exposure in the i filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.
We marginally detect a new source consistent with the FXT position (Lian et al., GCN Circ. 40429) and not visible in PanSTARRS at:
RA(J2000) = 13:28:26.68 = 202.1111 degrees Dec(J2000) = -2:42:11.1 = -2.7030 degrees with an uncertainty of 0.5 arcsec.
The preliminary magnitude derived for that source is:
i ~ 21.9
We note the presence of a possible host galaxy in deeper Legacy Survey images, located 0.5 arcsec from the optical counterpart candidate, with magnitudes of g = 23.61 +/- 0.04 and r = 23.67 +/- 0.05.
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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