GCN Circular 40455
Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, Tom Diehl (Fermilab), Haibin Zhang, Mitsuru Kokubo, Nozomu Tominaga, Yousuke Utsumi, Michitoshi Yoshida (NAOJ), Tomoki Morokuma (Chiba Tech), Akira Arai, Wanqui He, Yuki Moritani, Masato Onodera, Vera Maria Passegger, Ichi Tanaka, Kiyoto Yabe (NAOJ) report on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team, the Japanese Collaboration for Gravitational-Wave Electro-Magnetic Follow-up (J-GEM), and Subaru Telescope:
At 01:20 UTC April 6th and 00:14 UTC April 25th, the Dark Energy Camera (DECam) began the third and fourth epochs of observations in final response to the LVK alert issued for the candidate gravitational-wave event S250328ae (GCN 39898). The pointings of these observations were 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 ~21.3 in r-band, ~21.2 in i-band, and ~21.0 in z-band for the third epoch and ~22.3 in r-band, ~22.1 in i-band, and ~21.6 in z-band for the fourth.
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 at least 3 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. Of the 88 AGNs which exhibited transient variability in our observations, none lay within the localization volume.
Of the 25 high confidence candidates reported previously (GCN 39992), 15 were observed by the J-GEM collaboration, using the Subaru Telescope Prime Focus Spectrograph (PFS), and ruled out by spectral identification and redshift (GCN 40221). Another 8 candidates either did not exhibit any further transient activity after the first two epochs or were determined to be likely stellar in origin. The final 2 candidates were not within the footprint observed by J-GEM and thus could not be determined. We report these two candidates and one last candidate which was observed only in the most recent epochs below, all likely of supernova origin.
| TYPE | ID | ATNAME | RA | DEC | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
|---|---|---|---|---|---|---|---|---|---|---|
| SN_LIKE | 2290001 | AT2025avp | 144.806008 | 10.632223 | 20.36 | 0.02 | 20.19 | 0.02 | 20.22 | 0.06 |
| SN_LIKE | 2291473 | AT2025gem | 144.706509 | 11.560317 | 20.92 | 0.03 | 21.03 | 0.15 | 21.22 | 0.10 |
| SN_LIKE | 2295351 | AT2025kjv | 143.077064 | 7.385021 | 22.3 | 0.10 | 21.81 | 0.07 | 22.86 | 0.44 |
Additionally, J-GEM reported 5 QSOs which had a redshift consistent with the localization volume. We do not find any evidence of any transient variability for any of these QSOs in our observations after the detection of S250328ae. We also do not recover any of the X-ray source candidates reported by Swift XRT (GCN 39972) within their reported error bounds. Thus, apart from the possibility that the three supernova-like candidates reported above are related to S250328ae, we find no suitable optical counterpart candidate for this binary black hole gravitational wave event.
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. We also thank the J-GEM and Swift XRT teams for their contribution and support.
We are grateful to the staff at NAOJ and Subaru Telescope for their contributions to the deployments of PFS hardware and software, and the preparations of PFS system integration, engineering observations, and various other engineering works. Our thanks should also be propagated to the administrative staffs at Kavli IPMU, NAOJ, Subaru Telescope, and all the PFS institutes for kind support in such aspects as finances, contracts, asset managements, and so on. This research is based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawaii.