GCN Circular 39992

S. MacBride (UZH), I. McMahon (UZH), M. Soares-Santos (UZH), K. Herner (Fermilab), S. Kaur (University of Michigan/UZH), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:

At 01:46 UTC, March 30th, the Dark Energy Camera (DECam) began the second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250328ae (GCN 39898

These pointings are identical to the first epoch observations from March 29th, 2025 (GCN 39934

All fields were observed in DECam r, i, and z filters with 90-second exposures. The limiting magnitude achieved is ~22.8 in r-band, ~22.5 in i-band, and ~21.9 in z-band.

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, requiring an autoscan score of at least 0.7 on both nights of observations. 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.

After candidate selection, we report 25 high confidence candidates listed below. After vetting and identification, 15 candidates are classified as nuclear candidates (likely active galactic nuclei) and 10 candidates as possible supernovae. No candidates from the second epoch of observations match known AGNs within 1 arcsecond. 12 of the 20 candidates in the first epoch are listed here, while the candidates which do not appear did not have a confident detection in the second epoch. We also do not recover any of the X-ray source candidates reported by Swift XRT (GCN 39972

We encourage followup of the 25 candidates identified herein.

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.