TITLE: GCN CIRCULAR NUMBER: 41211 SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates DATE: 25/07/31 13:19:11 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, H. Thomas Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:
At 2025-07-29 09:39:10 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250727dc (GCN 41179). Observations covered the 90% localization region of the event (Prop ID: 2023B-851374; PI Andreoni/Palmese).
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. We 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, Quaia, and LQAC-6 AGN catalogs (Secrest et al. 2015, Flesch 2023, Storey-Fisher et al. 2024, Souchay et al. 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.
After candidate selection we report the 4 high confidence candidates listed below, including 2 nuclear candidates (likely AGN), and 1 likely supernova. Additionally, the transient AT2025ssi is the probable AGN Quaia_1070591 with a measured redshift of z~0.24 +- 0.08, which falls within the reported LVK volume. Further observations are ongoing, and we encourage followup of the 4 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 | 3334740 | AT2025ssf | 99.02238 | -36.026584 | 20.65 | 0.05 | 20.32 | 0.03 | N/A | N/A | 21.21 | 0.12 | | AGN_LIKE | 3334614 | AT2025ssg | 97.813775 | -36.075323 | 21.06 | 0.1 | 20.58 | 0.07 | N/A | N/A | 19.99 | 0.04 | | AGN_LIKE | 3334106 | AT2025ssh | 97.339644 | -36.600508 | 21.79 | 0.21 | N/A | N/A | 21.08 | 0.09 | 19.5 | 0.04 | | AGN_LIKE | 3334650 | AT2025ssi | 99.146548 | -37.499065 | 20.09 | 0.05 | N/A | N/A | 19.68 | 0.03 | N/A | N/A |
Additionally, we also recover two of the candidates reported by SWIFT-XRT (GCN 41201). One is the known blazar BL Lacertae (S250727dc_X1), which has been mentioned to be highly variable. The other (S250727dc_X7) is the galaxy ESO 365-5. Both of these galaxies have been spectroscopically measured to have redshifts z=0.055 (García-Pérez et al. 2024) and z=0.045 (Jones et al. 2009) respectively, which both lie outside of the LVK localization 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 | 3334338 | S250727dc_X1 | 96.778098 | -35.487631 | 17.32 | 0.01 | 16.98 | 0.01 | N/A | N/A | 16.5 | 0.0 | | AGN_LIKE | 3334206 | S250727dc_X7 | 94.688512 | -35.304098 | 18.79 | 0.02 | 18.32 | 0.02 | 18.34 | 0.01 | 18.51 | 0.02 |
We do not recover any of the other candidates reported by SWIFT-XRT, nor the transient AT2025smm reported by SWIFT-UVOT (GCN 41187).
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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