TITLE: GCN CIRCULAR NUMBER: 39191 SUBJECT: LIGO/Virgo/KAGRA S250206dm: SAGUARO Detection of Two Potential Optical Counterparts DATE: 25/02/07 05:55:46 GMT FROM: Griffin Hosseinzadeh at UC San Diego ghosseinzadeh@ucsd.edu

G. Hosseinzadeh, M. Shrestha, D. J. Sand, K. A. Bostroem, J. E. Andrews, B. Subrayan, J. Pearson, J. C. Rastinejad, M. J. Lundquist, C. D. Kilpatrick, W. Fong, K. Alexander, P. N. Daly, N. Franz, K. Paterson, S. D. Wyatt, J. Hogan, A. Gibbs, C. Fuls report on behalf of the SAGUARO collaboration:

We initiated observations of the localization region of the possible NSBH merger S250206dm (LVK Collaboration, GCN 39175) using the 5 square degree imager mounted on the 1.5m Catalina Sky Survey (CSS) telescope on Mt. Lemmon, AZ (Christensen et al. 2018). We observed 180 sq. deg. within the 95% confidence localization region of the Bilby map (LVK Collaboration, GCN 39178) starting at 2025-02-07 02:10:58 UTC (~5 hours after the merger). We subtract these images relative to deep reference images of each field and measure photometry calibrated to the Gaia DR2 catalog (Gaia Collaboration 2018).

We performed a real-time analysis of the observations. Following the methods outlined in Rastinejad et al. (2022), we crossmatch each candidate with the TNS (Gal-Yam 2021) and point source catalogs (Tachibana and Miller 2018; Jayasinghe et al. 2019; Flesch et al. 2021; Gaia Collaboration 2023), search public ZTF photometry (Bellm et al. 2019), and run ATLAS forced photometry (Shingles et al. 2021) at the position of the candidate to rule out transients unrelated to the GW event. We determine a most likely host galaxy using the probability of chance coincidence method (Bloom et al. 2002) and search for an associated spectroscopic or photometric redshift in public galaxy catalogs (White et al. 2011; Alam et al. 2015; Beck et al. 2016, 2021; Dalya et al. 2021; DESI Collaboration et al. 2023).

We discovered two possible transients in our images and reported them to the Transient Name Server. AT 2025azm (00:08:07.507 +32:33:56.70; 20.1 +/- 0.3 mag) is 1” from the center of a galaxy with a photometric redshift of z=0.07, corresponding to a distance of around 300 Mpc, in the Legacy Survey (Dey et al. 2019), SDSS (Alam et al. 2015), and Pan-STARRS1 (Chambers et al. 2016) catalogs. AT 2025azn (02:39:17.418 +49:34:21.10; 20.0 +/- 0.3 mag) is 0.5” from the center of a galaxy with a distance of 377 Mpc (z=0.08) in the GLADE catalog (Dalya et al. 2018). Both are consistent with the gravitational-wave distance within the error range.

We encourage follow-up photometry and spectroscopy of both these candidates. Our analysis is ongoing.

SAGUARO stands for Searches After Gravitational-waves Using ARizona's Observatories (Lundquist et al. 2019; Paterson et al. 2021; Hosseinzadeh et al. 2024). It is a partnership between the University of Arizona, Northwestern University, and the University of California, San Diego.

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