Transitions in the Mass-ratio and Spin Properties of Binary Black Holes in GWTC-5
Abstract
We analyze the mass-ratio and effective-spin (χeff) distributions of binary black hole mergers in the latest gravitational-wave catalog, GWTC-5, as a function of primary mass. Using hierarchical Bayesian inference with flexible Gaussian-process population models, we identify four distinct mass regions separated by sharp transitions in both mass-ratio and spin properties. Below ∼15 M⊙, the population strongly favors equal-mass binaries and exhibits a narrow χeff distribution peaked at positive values. In the range $18−30\,M_{\odot}$, the mass-ratio distribution becomes substantially flatter, while the χeff distribution broadens, shifts to a peak consistent with zero, and shows tentative--but not statistically required--evidence for positive skewness. The region associated with the feature near ≃35 M⊙ returns to a narrow χeff distribution consistent with symmetry at zero and strongly favors equal-mass binaries. Above ≃ 45 M⊙, both the mass-ratio and χeff distributions broaden significantly. The inferred support of the spin distribution converges toward the range expected for binaries containing remnants of previous black hole mergers, making the highest-mass region fully consistent with a star cluster population of hierarchical mergers. The close correspondence between transitions in mass ratio and effective spin suggests that different primary-mass ranges trace distinct formation channels, with isolated binary or triple evolution likely dominating the lower-mass population and dynamical assembly becoming increasingly important at higher masses.
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BibTeX
@misc{flanagan2026transitions,
title = {{Transitions in the Mass-ratio and Spin Properties of Binary Black Holes in GWTC-5}},
author = {Elizabeth Flanagan and Fabio Antonini and Thomas Callister and Debatri Chattopadhyay and Fani Dosopoulou and Isobel Romero-Shaw and Jakob Stegmann},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.14472},
eprint = {2606.14472},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.14472},
}
RIS
TY - UNPB
TI - Transitions in the Mass-ratio and Spin Properties of Binary Black Holes in GWTC-5
AU - Elizabeth Flanagan
AU - Fabio Antonini
AU - Thomas Callister
AU - Debatri Chattopadhyay
AU - Fani Dosopoulou
AU - Isobel Romero-Shaw
AU - Jakob Stegmann
PY - 2026
DA - 2026/10/07
PB - arXiv
DO - 10.48550/arxiv.2606.14472
UR - https://arxiv.org/abs/2606.14472
AB - We analyze the mass-ratio and effective-spin (χ_eff) distributions of binary black hole mergers in the latest gravitational-wave catalog, GWTC-5, as a function of primary mass. Using hierarchical Bayesian inference with flexible Gaussian-process population models, we identify four distinct mass regions separated by sharp transitions in both mass-ratio and spin properties. Below ∼15 M_⊙, the population strongly favors equal-mass binaries and exhibits a narrow χ_eff distribution peaked at positive values. In the range $18−30\,M_{\odot}$, the mass-ratio distribution becomes substantially flatter, while the χ_eff distribution broadens, shifts to a peak consistent with zero, and shows tentative--but not statistically required--evidence for positive skewness. The region associated with the feature near ≃35 M_⊙ returns to a narrow χ_eff distribution consistent with symmetry at zero and strongly favors equal-mass binaries. Above ≃ 45 M_⊙, both the mass-ratio and χ_eff distributions broaden significantly. The inferred support of the spin distribution converges toward the range expected for binaries containing remnants of previous black hole mergers, making the highest-mass region fully consistent with a star cluster population of hierarchical mergers. The close correspondence between transitions in mass ratio and effective spin suggests that different primary-mass ranges trace distinct formation channels, with isolated binary or triple evolution likely dominating the lower-mass population and dynamical assembly becoming increasingly important at higher masses.
ER -