Kavli Affiliate: Salvatore Vitale
| Summary:
The distribution of the effective inspiral spin ($χ_mathrmeff$) of the binary black holes detected by LIGO-Virgo-KAGRA can shed light on their formation pathways. We analyze the GWTC-5.0 dataset with two models-one flexible, one fully parametric-that jointly describe $χ_mathrmeff$ and primary mass. We clarify that the previously-reported skewness in the $χ_mathrmeff$ distribution is better understood as additional structure beyond a non-skewed Gaussian bulk centered at small $χ_mathrmeff$. This additional structure extends to larger $|χ_mathrmeff|$, a result previously reported using GWTC-4.0 data. We measure the asymmetry of the distribution of $χ_mathrmeff$ outside the Gaussian bulk from the data. With both the parametric and the flexible analyses, we find tentative evidence for a mass-dependent excess of positive $χ_mathrmeff$ over negative ones outside the Gaussian bulk. Only at $m_1 in [46,65],M_odot$ do the data require a negative $χ_mathrmeff$ component outside the Gaussian bulk, with $23text:1$ odds. If $χ_mathrmeff$ outside the Gaussian bulk are produced by hierarchical mergers-as it has been suggested-then a fraction of those mergers may be produced in environments that can generate a surplus of binaries with positive $χ_mathrmeff$, such as the disks of active galactic nuclei.
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