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Structure and Skewness of the Effective Inspiral Spin Distribution of Binary Black Hole Mergers

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arxiv 2501.06712 v2 pith:FJTW4TFC submitted 2025-01-12 astro-ph.HE gr-qc

Structure and Skewness of the Effective Inspiral Spin Distribution of Binary Black Hole Mergers

classification astro-ph.HE gr-qc
keywords populationspinsfeaturesbinaryblackchannelsdistributionfind
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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The detection of gravitational waves has brought to light a population of binary black holes that merge within a Hubble time. Multiple formation channels can contribute to this population, making it difficult to definitively associate particular population features with underlying stellar physics. Black hole spins are considered an important discriminator between various channels, but they are less well-measured than masses, making conclusive astrophysical statements using spins difficult thus far. In this paper, we consider the distribution of the effective inspiral spin $\chi_{\rm eff}$ -- a quantity much better measured than individual component spins. We show that non-Gaussian features like skewness, asymmetry about zero, and multimodality can naturally arise in the $\chi_{\rm eff}$ distribution when multiple channels contribute to the population. Searching for such features, we find signs of skewness and asymmetry already in the current catalogs, but no statistically significant signs of bimodality. These features provide robust evidence for the presence of a subpopulation with spins preferentially aligned to the binary's orbital angular momentum; and we conservatively estimate the fraction of this subpopulation to be at least $12 \% - 17\%$ (at $90\%$ credibility). Our models do not find an excess of non-spinning systems and instead find that at least $\sim 20 \%$ of the binaries have some degree of negative $\chi_{\rm eff}$. The data also suggest that, if preferentially aligned mergers form a significant fraction of the population, they must have small spins.

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Cited by 10 Pith papers

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  3. Signatures of a subpopulation of hierarchical mergers in the GWTC-4 gravitational-wave dataset

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    Using a joint effective-spin and precession-spin model on 155 gravitational-wave events, the authors infer that the hierarchical (second-generation) merger fraction rises sharply above ~46 M_sun and peaks again near 1...

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  8. Evidence for additional structure in the effective spin distribution hints at multiple formation pathways in GWTC-5.0

    astro-ph.HE 2026-06 unverdicted novelty 4.0

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    A review summarizing formation-channel predictions, waveform effects, and population-level constraints on stellar-mass black hole spins from the first decade of gravitational-wave observations.