TITLE:   GCN CIRCULAR
NUMBER:  39468
SUBJECT: LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: DECam GW-MMADS candidates
DATE:    25/02/25 06:46:50 GMT
FROM:    xjh@andrew.cmu.edu

Lei Hu (CMU), Xander J. Hall (CMU), Tomás Cabrera (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Brendan O’Connor (CMU), Keerthi Kunnumkai (CMU), 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 by the Dark Energy Survey Gravitational Wave (DESGW) program (PI: Soares-Santos; GCN 39462). Observations started at 2025-02-24T03:45:38 on the night of the alert, and at 2025-02-25T00:50:13 on the following night. We find that the median 5sigma depths of the exposures are r\~24.7 mag on the first night and r\~24.9 mag on the second night. 

As part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS), we run the SFFT difference imaging (Hu et al. 2022) on the available images using templates from DES, filter out likely stars and moving objects, visually inspect the remaining transients. We posted on TNS new transients from this analysis and report here the most likely extragalactic transients we find within the joint LVK-GUANO skymap 95% credible interval area:

| id | AT name | ra | dec | discovery_date (UT) | mag_r | mag_r_err | mag_r-mag_i |

| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- |
| C202502240542297m473928 | AT 2025cqb | 85.623700 | -47.657680 | 2025-02-24T03:58:42 | 23.04 | 0.05 | 0.20
| C202502240549253m474247 | AT 2025cqf | 87.355530 | -47.713117 | 2025-02-24T03:58:42 | 24.44 | 0.20 | N/A
| A202502240541155m473906* | AT 2025cpv | 85.314435 | -47.651722 | 2025-02-24T03:45:38 | 23.55 | 0.08 | 0.21
| C202502240541459m473517 | AT 2025cql | 85.441205 | -47.588088 | 2025-02-24T04:24:50 | 24.77 | 0.25 | N/A
| C202502240545203m483410 | AT 2025cqm | 86.334777 | -48.569306 | 2025-02-24T03:58:42 | 24.00 | 0.13 | N/A
| T202502240543468m475332 | AT 2025cqc | 85.944938 | -47.892308 | 2025-02-24T03:45:38 | 23.00 | 0.05 | 0.21
| C202502240544410m483014 | AT 2025cqe | 86.170896 | -48.503829 | 2025-02-24T03:58:42 | 23.75 | 0.10 | 0.49
| A202502240546005m475901* | AT 2025cqd | 86.502090 | -47.983653 | 2025-02-24T03:45:38 | 22.48 | 0.03 | 0.23

* Possible stellar origin

Magnitudes reported are from the discovery date and not corrected from Milky Way extinction.

Further analysis is underway.

We thank the CTIO and NOIRLab staff for supporting observations and data calibration.