TITLE: GCN CIRCULAR NUMBER: 39193 SUBJECT: LIGO/Virgo/KAGRA S250206dm: COLIBRÍ/DDRAGO Observations of the AT 2025azn Optical Candidate DATE: 25/02/07 08:33:03 GMT FROM: Alan Watson at UNAM alan@astro.unam.mx

Alan M. Watson (UNAM), Jean-Grégoire Ducoin (CPPM), Sarah Antier (OCA), Stéphane Basa (UAR Pytheas), Jean-Luc Atteia (IRAP), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Rosa L. Becerra (Università degli Studi di Roma Tor Vergata), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), and Benjamin Schneider (LAM) report:

We imaged the field of the AT 2025azn candidate reported by the SAGUARO team (GCN Circ. 39191) as a possible optical counterpart of the GW transient S250206dm (GCN Circ. 29175, GCN Circ. 39184) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.

We observed from 2025-02-07 06:39 to 07:27 UTC (9.24 to 10.04 hours after the GW transient), with a midpoint of 9.64 hours after the event, and obtained 840 seconds of exposure in the g filter and 840 seconds of exposure in the r filter in good weather conditions, albeit at airmass 2.1 to 2.8. The data were reduced and stacked using custom software and then calibrated against the PS1 catalog and analysed using STDPipe (Karpov 2021). The FWHM in both stacks is about 2.0 arcsec.

At the position of AT 2025azn, and after template subtraction using Pan-STARRS DR2 reference images, we do not detect any source at a 3 sigma limiting magnitudes of:

g > 21.9

r > 21.7

Our magnitude limits are not corrected for Galactic extinction.

These limits are significantly fainter than the detection reported by the SAGUARO team from observation taken about 2 hours earlier.

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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