TITLE: GCN CIRCULAR NUMBER: 40203 SUBJECT: EP250421A: COLIBRÍ Optical Upper Limit DATE: 25/04/22 08:13:37 GMT FROM: Rosa L. Becerra at Tor Vergata, Roma rosa.becerra@roma2.infn.it

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

We imaged the field of EP250421A (Zhao et al., GCN Circ. 40198) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-22 UTC.

We observed from 2025-04-22 04:54 to 06:03 UTC (T+12.6 to T+13.8 hours after the trigger) and obtained 50 minutes of exposure in total in the r filter. Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.

At the position of the optical counterpart (Lee et al., GCN Circ. 40192; Fu et al., GCN Circ. 40193; Liu et al., GCN Circ. 40197), we obtained an upper limit (5-sgima) of:

r > 23.44

We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.

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