TITLE: GCN CIRCULAR NUMBER: 39497 SUBJECT: LIGO/Virgo/KAGRA S250223dk: DECam optical detections of Swift X-ray sources DATE: 25/02/26 17:49:58 GMT FROM: Antonella Palmese at Carnegie Mellon University <apalmese@andrew.cmu.edu> Antonella Palmese (CMU), Lei Hu (CMU), Xander J. Hall (CMU), Tomás Cabrera (CMU), Igor Andreoni (UNC), Brendan O’Connor (CMU), Keerthi Kunnumkai (CMU), report on behalf of the GW-MMADS team: The high probability area of the joint LVK/Swift-GUANO alert for the gravitational wave candidate S250223dk (GCN 39443) was observed using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope (GCN 39462), and we analyzed the data from observations starting 2025-02-24T03:45:38 and 2025-02-25T00:50:13 (GCN 39468). We detect a variable source consistent with the location of the Swift XRT source S250223dk_X1 (GCN 39485) at the position of a Quaia (Storey-Fisher et al. 2024) likely quasar source (unWISE source id 0850m470o0042962, RA, dec=85.499,-47.358 [J2000]) with a photometric redshift of 1.4+-0.2, consistent with the distance of the GW alert. Stacked difference imaging with respect to templates from the Dark Energy Survey data reveals a clear source (AT 2025csi) in the griz bands. Our preliminary photometry on the difference imaging results in the following magnitudes: | MJD | Band | mag | | ---------- | ---- | -------------- | | 60730.1567 | g | 20.90 +- 0.01 | | 60730.1748 | i | 20.38 +- 0.01 | | 60731.0349 | g | 20.82 +- 0.007 | | 60731.0530 | i | 20.41 +- 0.009 | We also inspected archival DECam data at this location and note that this possible quasar showed a steady ~1 mag increase in brightness in all bands between 2013 and 2018. Some archival detections are also present from ATLAS forced photometry over the past two years. The archival detections suggest that the source variability may be unrelated to the GW alert, although we encourage follow-up observations to establish the nature of this object and whether it is currently experiencing flaring activity. We also detect a transient (AT 2025cpu) in the griz bands consistent with the location of S250223dk_X5 at position RA, dec= 85.51643, -47.70934 [J2000], with the following magnitudes: | MJD | Band | mag | | ---------- | ---- | ------------- | | 60730.1569 | g | 22.71 +- 0.03 | | 60730.1748 | i | 22.63 +- 0.07 | | 60731.0349 | g | 22.55 +- 0.03 | | 60731.0530 | i | 22.74 +- 0.05 | We note that this source appears as a nuclear transient in a possible galaxy with photometric redshift of 0.797+-0.088 (from the DESI Legacy Survey photometric redshift catalog). The archival DECam observations for this source show variability of up to a magnitude, thus we cannot exclude that these detections are due to AGN variability. Further analysis of these sources is planned and follow-up observations are encouraged. We thank the CTIO and NOIRLab staff for supporting observations and data calibration. View this GCN Circular online at https://gcn.nasa.gov/circulars/39497. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...