Transitions in the mass-ratio and spin properties of binary black holes in GWTC-5

Flanagan, Elizabeth, Antonini, Fabio ORCID: https://orcid.org/0000-0003-3138-6199, Callister, Thomas, Chattopadhyay, Debatri, Dosopoulou, Fani, Romero-Shaw, Isobel ORCID: https://orcid.org/0000-0002-4181-8090 and Stegmann, Jakob 2026. Transitions in the mass-ratio and spin properties of binary black holes in GWTC-5. The Astrophysical Journal Letters 1011 (2) , L53. 10.3847/2041-8213/aeb120

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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 transitions in both mass-ratio and spin properties. Below ∼18 M⊙ the population strongly favors equal-mass binaries and exhibits a narrow, symmetric χeff distribution peaked at positive values. The positive skewness previously reported for the overall population is absent from this low-mass component, and it is most likely associated with higher-mass binaries. In the range 18–30M⊙, the mass-ratio distribution becomes substantially flatter, while the χeff distribution broadens and shifts to a peak consistent with zero. 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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physics and Astronomy Schools > Physical, Chemical & Environmental Sciences
Publisher:	American Astronomical Society
ISSN:	2041-8205
Date of First Compliant Deposit:	12 November 2026
Date of Acceptance:	6 October 2026
Last Modified:	12 Nov 2026 10:45
URI:	https://orca.cardiff.ac.uk/id/eprint/190880

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