TITLE: GCN CIRCULAR NUMBER: 41206 SUBJECT: LIGO/Virgo/KAGRA S250725j: DECam DESGW Epoch 1-2 Candidates DATE: 25/07/31 00:52:54 GMT FROM: Isaac McMahon at University of Zürich isaac.mcmahon@ligo.org

Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, Tom Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 2025-07-25 23:13:43 UTC and 2025-07-28 23:13:54 UTC, the Dark Energy Camera (DECam) began the first and second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250725j (GCN 41154). We observed nine fields centered on the following ICRS coordinates: | RA | DEC | |----------|----------| | 231.833836 | -37.23415| |234.585764 | -35.2584| |232.520774 | -35.64614| |233.935432 | -36.84845| |236.026051 | -36.42542| |230.4476 | -35.99735| |233.180871 | -34.0649| |236.640727 | -34.83446| |229.723284 | -37.58219|

These pointings cover the 90% localization region of candidate gravitational-wave event S250725j.

All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 23.2 in g, 22.9 in r, and 22.6 in i, and 21.6 in z. The limiting magnitudes achieved for epoch 2 were 22.5 in g, 22.6 in r, and 22.3 in i, and 21.6 in z.

We process the images with 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. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.

After candidate selection we report the 3 high confidence candidates listed below, including 2 nuclear candidates (likely active galactic nuclei) and 1 likely supernova. Further observations are ongoing, and we encourage followup of the 3 candidates identified herein.

| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |---------- |--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- | | SN_LIKE | 3141695 | AT2025srk | 234.755524 | -36.425993 | 21.51 | 0.02 | 21.0 | 0.02 | 21.27 | 0.03 | 22.06 | 0.27 | | AGN_LIKE | 3237805 | AT2025srl | 234.90473 | -36.726032 | 23.55 | 0.19 | N/A | N/A | 19.74 | 0.01 | N/A | N/A | | AGN_LIKE | 3141486 | AT2025srm | 232.987656 | -36.816564 | 22.43 | 0.06 | 22.07 | 0.04 | 22.74 | 0.17 | 21.21 | 0.05 |

Additionally, we also recover two of the candidates reported by GW-MMADS (GCN 41177). The candidate AT2025sib is of particular note, as its photometric redshift reported by 2MASS (Bilicki et al 2014) lies within the LVK 90% volume.

| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |---------- |--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- | | AGN_LIKE | 3141729 | AT2025shz | 231.072453 | -37.6226 | 22.77 | 0.08 | 22.36 | 0.08 | N/A | N/A | N/A | N/A | | SN_LIKE | 3141898 | AT2025sib | 235.548953 | -35.002893 | 21.72 | 0.04 | 20.7 | 0.02 | N/A | N/A | 20.72 | 0.07 |

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.

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