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.