TITLE: GCN CIRCULAR NUMBER: 40221 SUBJECT: LIGO/Virgo/KAGRA S250328ae: Subaru/PFS Spectroscopic Follow up and Candidates DATE: 25/04/24 05:12:11 GMT FROM: Haibin Zhang at National Astronomical Observatory of Japan (NAOJ) haibin.zhang@nao.ac.jp

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), Sean MacBride, Isaac McMahon, Marcelle Soares-Santos (UZH), Ken Herner (Fermilab), Simran Kaur (University of Michigan/UZH), Lillian Joseph (Benedictine U.), and Tom Diehl (FNAL) report on behalf of the Japanese Collaboration for Gravitational-Wave Electro-Magnetic Follow-up (J-GEM), Subaru Telescope, and Dark Energy Survey Gravitational Wave (DESGW) Team:

Between 05:11 and 10:20 UTC on April 3, 2025, we carried out spectroscopic observations with the Subaru/Prime Focus Spectrograph (PFS) following the LVK alert issued for the gravitational-wave event S250328ae (GCN 39898). The Subaru/PFS is a fiber spectrograph capable of observing ~2400 targets simultaneously within a ~1.25 square degree field of view, and covers a wavelength range of 380 nm to 1260 nm with a resolving power of ~2500-5500 (Sugai et al. 2015; Tamura et al. 2024). We observed seven pointings centered on the ICRS coordinates listed below:

|RA [deg]|Dec [deg]| | - | - | |144.130530|10.436230| |144.126501|11.595492| |145.151367|9.856599| |145.151367|11.015861| |145.151367|12.175123| |146.176234|11.595492| |146.180717|12.754753|

These seven pointings cover the ~50% localization region of the S250328ae event.

We observed targets selected from transient candidates of GCN 39934 and 39992 (including candidates before and after vetting), X-ray sources from GCN 39972, and potential host galaxies from GLADE+ (Dálya et al. 2022) and PS1-STRM (Beck et al. 2021) catalogs in our pointings. A total of ~3900 targets were observed with an on-source exposure time of 1800 seconds in six pointings and 900 seconds in one pointing (shortened due to bad weather conditions).

We reduced the data on-site using the quick reduction system based on the PFS Data Reduction Pipeline, and then carried out classification (fitting galaxy, QSO, star, and supernova templates) and visual inspection. After classification and visual inspection, we obtained confident spectroscopic redshifts of ~70% of our targets. Among these sources, five candidates listed below were identified to be the possible electromagnetic counterpart to the gravitational-wave event S250328ae (within the ~90% three-dimensional localization volume partly covered by our pointings). Because these five candidates were originally selected from the GLADE+ and PS1-STRM catalogs that do not contain information of variability, we encourage follow-up observations to confirm their variability.

|ID_PFS|ATNAME|RA [deg]|Dec [deg]|Type_PFS|Redshift| | - | - | - | - | - | - | |761|AT2019uib|145.718857|12.412288|QSO|0.130| |9238|N/A|145.412644|12.717089|QSO|0.096| |13647|N/A|144.989553|10.636089|QSO|0.136| |17165|N/A|145.733559|11.593362|QSO|0.120| |19826|N/A|145.351893|12.503455|QSO|0.136|

No counterparts to X-ray sources (GCN 39972) are recovered within their reported error bounds by our observations. We do not identify any transient candidates (GCN 39934 and 39992) we observed within the ~90% three-dimensional localization volume of S250328ae. These transient candidates falling outside the ~90% localization volume are listed below.

|ID_ DESGW|ATNAME|RA [deg]|Dec [deg]|Type_ DESGW|Type_PFS|Redshift| | - | - | - | - | - | - | - | |2290036|AT2025gek|145.199481|10.828527|SN_LIKE|SN Ic|0.233| |2290467|AT2020woa|144.388798|10.255536|SN_LIKE|SN Ib|0.050| |2292782|AT2025geo|144.928578|10.349243|SN_LIKE|SN Ia|0.182| |2290334|AT2025geq|145.235616|12.469694|SN_LIKE|SN Ia|0.186| |2290623|AT2025gen|144.112464|11.641817|AGN_LIKE|SN II|0.204| |2291223|N/A|146.389563|13.131801|AGN_LIKE|QSO|0.540| |2293190|AT2025cvb|144.191297|10.828462|SN_LIKE|SN Ia|0.140| |2290143|AT2025ggv|146.033906|11.088812|SN_LIKE|QSO|1.437| |2292040|AT2025ges|144.438216|9.885617|AGN_LIKE|SN Ia|0.434| |2290059|N/A|144.679961|10.018211|AGN_LIKE|QSO|0.209| |2292786|N/A|144.037664|10.810454|AGN_LIKE|SN Ia|0.254| |2293517|AT2025geu|145.956024|12.381673|AGN_LIKE|QSO|0.507| |2295862|N/A|145.361115|12.521385|AGN_LIKE|QSO|0.969| |2290563|N/A|145.149074|11.756403|AGN_LIKE|QSO|0.255| |2289995|AT2025ggx|144.57373|10.274445|SN_LIKE|SN II|0.358|

We are grateful to the staffs 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 supports 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 the cultural, historical, and natural significance in Hawaii.

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.

View this GCN Circular online at https://gcn.nasa.gov/circulars/40221. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...