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1. SOAR/Goodman Spectroscopic Assessment of Candidate Counterparts of the LIGO/Virgo Event GW190814*

   https://doi.org/10.3847/1538-4357/ac5b60  

   Tucker, D. L.; Wiesner, M. P.; Allam, S. S.; Soares-Santos, M.; Bom, C. R.; Butner, M.; Garcia, A.; Morgan, R.; Olivares E., F.; Palmese, A.; et al (April 2022, The Astrophysical Journal)

   Abstract On 2019 August 14 at 21:10:39 UTC, the LIGO/Virgo Collaboration (LVC) detected a possible neutron star–black hole merger (NSBH), the first ever identified. An extensive search for an optical counterpart of this event, designated GW190814, was undertaken using the Dark Energy Camera on the 4 m Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory. Target of Opportunity interrupts were issued on eight separate nights to observe 11 candidates using the 4.1 m Southern Astrophysical Research (SOAR) telescope’s Goodman High Throughput Spectrograph in order to assess whether any of these transients was likely to be an optical counterpart of the possible NSBH merger. Here, we describe the process of observing with SOAR, the analysis of our spectra, our spectroscopic typing methodology, and our resultant conclusion that none of the candidates corresponded to the gravitational wave merger event but were all instead other transients. Finally, we describe the lessons learned from this effort. Application of these lessons will be critical for a successful community spectroscopic follow-up program for LVC observing run 4 (O4) and beyond. 

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2. Search for neutrinos in coincidence with gravitational wave events from the LIGO-Virgo O3a Observing Run with the Super-Kamiokande detector

   The Super-Kamiokande Collaboration (January 2021, The Astrophysical journal)

   The Super-Kamiokande detector can be used to search for neutrinos in time coincidence with gravitational waves detected by the LIGO–Virgo Collaboration (LVC). Both low-energy (7–100 MeV) and high-energy (0.1–105 GeV) samples were analyzed in order to cover a very wide neutrino spectrum. Follow-ups of 36 (out of 39) gravitational waves reported in the GWTC-2 catalog were examined; no significant excess above the background was observed, with 10 (24) observed neutrinos compared with 4.8 (25.0) expected events in the high-energy (low-energy) samples. A statistical approach was used to compute the significance of potential coincidences. For each observation, p-values were estimated using neutrino direction and LVC sky map; the most significant event (GW190602_175927) is associated with a post-trial p-value of 7.8% (1.4σ). Additionally, flux limits were computed independently for each sample and by combining the samples. The energy emitted as neutrinos by the identified gravitational wave sources was constrained, both for given flavors and for all flavors assuming equipartition between the different flavors, independently for each trigger and by combining sources of the same nature. 

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3. AT2025ulz and S250818k: Leveraging DESI Spectroscopy in the Hunt for a Kilonova Associated with a Subsolar-mass Gravitational-wave Candidate

   https://doi.org/10.3847/2041-8213/ae4338  

   Hall, Xander J; Palmese, Antonella; O’Connor, Brendan; Gruen, Daniel; Busmann, Malte; Gassert, Julius; Hu, Lei; Magaña_Hernandez, Ignacio; Aguilar, Jessica Nicole; Amsellem, Ariel; et al (April 2026, The Astrophysical Journal Letters)

   Abstract On 2025 August 18, the LIGO–Virgo–KAGRA collaboration reported a subthreshold gravitational-wave candidate detection consistent with a subsolar-mass neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova emission, the telltale signature of a binary neutron star merger. The transient subsequently rebrightened after ∼5 days and was classified as a Type IIb supernova. In this work, we analyze the observations of its host galaxy obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure redshift ofz = 0.084840 ± 0.000006. If S250818k has an astrophysical origin, this places the transient within 2σof the gravitational-wave distance and results in an integral overlap between the gravitational-wave alert and the transient location of ${\log }_{10}\mathcal{I}\approx 3.9-4.2$ . Our analysis of the host galaxy’s spectral energy distribution reveals a star-forming, dusty galaxy with stellar mass ∼10¹⁰M_⊙, broadly consistent with the population of both short gamma-ray bursts and core-collapse supernova host galaxies. We also present our follow-up of DESI-selected candidate host galaxies using the Fraunhofer Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational-wave alerts. These results emphasize the value of DESI’s extensive spectroscopic dataset in rapidly characterizing host galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up. 

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4. GWTC-4.0: Methods for Identifying and Characterizing Gravitational-wave Transients

   https://doi.org/10.3847/2041-8213/ae447b  

   Abac, A G; Abouelfettouh, I; Acernese, F; Ackley, K; Adhicary, S; Adhikari, D; Adhikari, N; Adhikari, R X; Adkins, V K; Afroz, S; et al (June 2026, The Astrophysical Journal Letters)

   Abstract The Gravitational-Wave Transient Catalog (GWTC) is a collection of candidate gravitational-wave transient signals identified and characterized by the LIGO–Virgo–KAGRA Collaboration. Producing the contents of the GWTC from detector data requires complex analysis methods. These comprise techniques to model the signal; identify the transients in the data; evaluate the quality of the data and mitigate possible instrumental issues; infer the parameters of each transient; compare the data with the waveform models for compact binary coalescences; and handle the large amount of results associated with all of these different analyses. In this paper, we describe the methods employed to produce the catalog’s fourth release, GWTC-4.0, focusing on the analysis of the first part of the fourth observing run of Advanced LIGO, Advanced Virgo and KAGRA. 

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5. Implications of the search for optical counterparts during the second part of the Advanced LIGO’s and Advanced Virgo’s third observing run: lessons learned for future follow-up observations

   https://doi.org/10.1093/mnras/staa1925  

   Coughlin, Michael W; Dietrich, Tim; Antier, Sarah; Almualla, Mouza; Anand, Shreya; Bulla, Mattia; Foucart, Francois; Guessoum, Nidhal; Hotokezaka, Kenta; Kumar, Vishwesh; et al (September 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Joint multimessenger observations with gravitational waves and electromagnetic (EM) data offer new insights into the astrophysical studies of compact objects. The third Advanced LIGO and Advanced Virgo observing run began on 2019 April 1; during the 11 months of observation, there have been 14 compact binary systems candidates for which at least one component is potentially a neutron star. Although intensive follow-up campaigns involving tens of ground and space-based observatories searched for counterparts, no EM counterpart has been detected. Following on a previous study of the first six months of the campaign, we present in this paper the next five months of the campaign from 2019 October to 2020 March. We highlight two neutron star–black hole candidates (S191205ah and S200105ae), two binary neutron star candidates (S191213g and S200213t), and a binary merger with a possible neutron star and a ‘MassGap’ component, S200115j. Assuming that the gravitational-wave (GW) candidates are of astrophysical origin and their location was covered by optical telescopes, we derive possible constraints on the matter ejected during the events based on the non-detection of counterparts. We find that the follow-up observations during the second half of the third observing run did not meet the necessary sensitivity to constrain the source properties of the potential GW candidate. Consequently, we suggest that different strategies have to be used to allow a better usage of the available telescope time. We examine different choices for follow-up surveys to optimize sky localization coverage versus observational depth to understand the likelihood of counterpart detection. 

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