Eccentricity Constraints Disfavor Single–Single Capture in Nuclear Star Clusters as the Origin of all LIGO–Virgo–KAGRA Binary Black Holes

Year: 2026

Type: article

Abstract: Multiple formation pathways have been proposed for the origin of binary black holes (BBHs). These include isolated binary evolution and dynamical assembly in dense stellar environments such as nuclear or globular star clusters. Yet, the fraction of BBHs originating from each channel remains uncertain. One way to constrain this fraction is by investigating the orbital eccentricities of the black hole coalescences detected by the LIGO–Virgo–KAGRA (LVK) Collaboration. We analyze 84 BBHs from the first part of the fourth LVK observing run (O4a) using a multipolar, eccentric, aligned-spin effective-one-body waveform model. We perform parameter inference with neural posterior estimation and nested sampling. After incorporating astrophysical prior odds and comparing to the quasicircular precessing-spin hypothesis, we find that no candidates reach a high enough significance to claim a confident detection of eccentricity. We use these upper limits to explore a model in which all O4a BBHs originate from single–single gravitational-wave (GW) captures. We perform hierarchical inference on the velocity dispersion of the host environment of the BBHs and find σ < 19.7 km s −1 (95% credible upper bound). This disfavors single–single capture in nuclear star clusters (∼20–200 km s −1 ) as the dominant source of all observed BBH mergers. Our analysis also jointly infers the mass, spin, and redshift distributions and takes into account selection effects due to using quasicircular templates for detection. Our results place improved constraints on the number of eccentric BBHs and highlight the importance of eccentricity measurements in disentangling compact-binary formation channels in current and future GW detectors.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/ae9479

Authors: Nihar Gupte, M. Coleman Miller, Rhiannon Udall, Sophie Bini, Alexandre Toubiana, Alessandra Buonanno, Jonathan Gair, Aldo Gamboa, Lorenzo Pompili, Antoni Ramos-Buades, Maximilian Dax, Stephen R. Green, Annalena Kofler, Jakob Macke, Bernhard Schölkopf

Institutions: Max Planck Institute for Gravitational Physics, University of Maryland, College Park, California Institute of Technology, University of British Columbia, University of Milano-Bicocca, University of Nottingham, Universitat de les Illes Balears, Max Planck Institute for Intelligent Systems, Tübingen AI Center, ELLIS Institute Tübingen, University of Tübingen

Language: English

Topic: Pulsars and Gravitational Waves Research

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $4,499

Funders: National Science Foundation, UK Research and Innovation, Ministerio de Ciencia, Innovación y Universidades, International Max Planck Research School for Environmental, Cellular and Molecular Microbiology, European Commission, National Research Foundation, Ministry of Education, Culture, Sports, Science and Technology, Academia Sinica, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Centre National de la Recherche Scientifique, Universitat de les Illes Balears, International Max Planck Research School for Advanced Methods in Process and Systems Engineering, Istituto Nazionale di Fisica Nucleare, Ministry of Science and ICT, South Korea, National Science and Technology Council, Science and Technology Facilities Council, Japan Society for the Promotion of Science, European Regional Development Fund, Agencia Estatal de Investigación

OpenAlex ID: W7147014794