TITLE:   GCN CIRCULAR
NUMBER:  39462
SUBJECT: LIGO/Virgo/KAGRA S250223dk: DECam DESGW Candidates
DATE:    25/02/24 22:36:16 GMT
FROM:    Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>

I. McMahon (UZH), S. MacBride (UZH), H. T. Diehl (FNAL), S. Kaur (UZH), L. Joseph (Benedictine U.), N. Sherman (Boston U.), K. Herner (Fermilab), M. Soares-Santos (UZH), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 03:45:38 UTC, the Dark Energy Camera (DECam) began observing in response to the joint LVK/Swift-BAT RAVEN alert issued for the candidate gravitational-wave event S250223dk  (GCN 39443). We observed two fields centered on the following ICRS coordinates
(85.80,-47.60)
(85.83,-47.63)
Both fields were observed in DECam g, r, i, and z filters with 360 second exposures. The limiting magnitude achieved is 23.36 in r-band. 

Images were processed by our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al. 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. 

After candidate selection, we report five high confidence candidates (listed below). After vetting and identification, four candidates were classified as nuclear candidates (likely AGNs), one candidate (2017239) has been labeled as a possible supernova. All candidates have a host galaxy match within 1 arcsecond. None of the candidate hosts are included in the ALLWISE and MILLIQUAS AGN catalogs (Secrest et. al 2015, Flesch 2023). No other candidates were found in the area. We encourage followup of the five candidates identified herein. 

| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_r | mag_r_err | mag_i | mag_i_err | mag_z | mag_z_err |
| --------------- | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ------ | ----- | ------ | ----- | ------ | ----- |
| 2014974 | AT2025cpl | 86.327994 | -47.827215 | 2025-02-24 03:45:38.271 | 22.487 | 0.043 | 22.151 | 0.031 | 21.744 | 0.053 | 21.894 | 0.106 |
| 2014991 | AT2025cpm | 86.516403 | -47.893872 | 2025-02-24 03:45:38.271 | 23.448 | 0.107 | 22.905 | 0.060 | 22.453 | 0.103 | 22.308 | 0.153 |
| 2015571 | AT2025cpo | 86.105565 | -47.689987 | 2025-02-24 03:45:38.271 | 22.972 | 0.053 | N/A | N/A | 22.423 | 0.084 | 22.765 | 0.203 |
| 2015600 | AT2025cpp | 86.176728 | -47.763957 | 2025-02-24 03:52:10.108 | 21.980 | 0.024 | N/A | N/A | 22.097 | 0.061 | 22.312 | 0.132 |
| 2017239 | AT2025cpq | 85.552523 | -47.577823 | 2025-02-24 03:58:42.138 | 24.503 | 0.268 | 23.980 | 0.161 | 23.972 | 0.320 | 24.265 | 0.851 |

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.