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

Abstract

The distribution of the effective inspiral spin ($\ddot{I}{‡}_{\mathrm{eff}}$) 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 $\ddot{I}{‡}_{\mathrm{eff}}$ and primary mass. We clarify that the previously-reported skewness in the $\ddot{I}{‡}_{\mathrm{eff}}$ distribution is better understood as additional structure beyond a non-skewed Gaussian bulk centered at small $\ddot{I}{‡}_{\mathrm{eff}}$. This additional structure extends to larger $|\ddot{I}{‡}_{\mathrm{eff}}|$, a result previously reported using GWTC-4.0 data. We measure the asymmetry of the distribution of $\ddot{I}{‡}_{\mathrm{eff}}$ outside the Gaussian bulk from the data. With both the parametric and the flexible analyses, we find suggestive evidence for a mass-dependent excess of positive $\ddot{I}{‡}_{\mathrm{eff}}$ over negative ones outside the Gaussian bulk. We find only a mild statistical preference for a negative $\ddot{I}{‡}_{\mathrm{eff}}$ component outside the Gaussian bulk. This preference is highest at $m_1\in [47,65]M_{\odot }$, with $13\text{:}1$ odds. Excluding the event GW191109, whose data have known data-quality issues, reduces these odds to $5\text{:}1$. If $\ddot{I}{‡}_{\mathrm{eff}}$ outside the Gaussian bulk are produced by hierarchical mergers-as has been suggested-then a fraction of those mergers may be produced in environments that can generate a surplus of binaries with positive $\ddot{I}{‡}_{\mathrm{eff}}$, such as the disks of active galactic nuclei.

Letter: 5 pages, 4 figures. Supplementary Material: 4 pages, 2 figures. Updated to match the version submitted to the journal