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Abstract The complex astrophysical processes leading to the formation of binary black holes and their eventual merger are imprinted on the spins of the individual black holes. We revisit the astrophysical distribution of those spins based on gravitational waves from the third Gravitational-wave Transient Catalog, looking for structure in the two-dimensional space defined by the dimensionless spin magnitudes of the heavier (χ₁) and lighter (χ₂) component black holes. We find support for two distinct subpopulations with greater than 95% credibility. The dominant population is made up of black holes with small spins, preferringχ₁≈ 0.2 for the primary andχ₂≈ 0 for the secondary; we report signs of an anticorrelation betweenχ₁andχ₂, as well as evidence against a subpopulation of binaries in which both components are nonspinning. The subdominant population consists of systems in which both black holes have relatively high spins and contains $20_{-18}^{+18}\%$ of the binaries. The binaries most likely to belong in this subpopulation are massive and slightly more likely to have spin orientations aligned with the orbital angular momentum—potentially consistent with isolated binary formation channels capable of producing large spins, like chemically homogeneous evolution. This hint of a rapidly spinning subpopulation hinges on GW190517, a binary with large, well-measured spins. Our results, enabled by novel hierarchical inference methods, represent a first step towards more descriptive population models for black hole spins and will be strengthened or refuted by the large number of gravitational-wave detections expected in the next several years. 

Abstract GW231123, the most massive binary black hole (BH) merger observed to date, involves component BHs with masses inside the pair-instability mass gap and unusually high spins. This challenges standard formation channels such as classical stellar evolution and hierarchical mergers. However, stellar rotation and magnetic fields, which have not been systematically incorporated in prior models, can strongly influence the BH properties. We present the first self-consistent simulations tracking a massive, low-metallicity helium star from helium core burning through collapse, BH formation, and post-BH formation accretion using 3D general-relativistic magnetohydrodynamic simulations. Starting from a 250M_⊙helium core, we show that collapse above the pair-instability mass gap, aided by rotation and magnetic fields, drives mass loss through disk winds and jet launching. This enables the formation of highly spinning BHs within the mass gap and reveals a BH spin–mass correlation. Strong magnetic fields extract angular momentum from the BH through magnetically driven outflows, which in turn suppress accretion, resulting in slowly spinning BHs within the mass gap. In contrast, stars with weak fields permit nearly complete collapse and spin-up of the BH toa ≈ 1. We show that massive low-metallicity stars with moderate magnetic fields naturally produce BHs whose masses and spins match those inferred for GW231123, and are also consistent with those of GW190521. The outflows launched during collapse may impart a BH kick, which can induce spin–orbit misalignment and widen the postcollapse orbit, delaying the merger. These outflows could further drive short-lived, high-luminosity jets comparable to the most energeticγ-ray bursts, offering a potential observational signature of such events in the early Universe. 

Abstract Gravitational-wave observations of binary black holes have revealed unexpected structure in the black hole mass distribution. Previous studies employ physically motivated phenomenological models and infer the parameters that control the features of the mass distribution that are allowed in their model, associating the constraints on those parameters with their physical motivations a posteriori. In this work, we take an alternative approach in which we introduce a model parameterizing the underlying stellar and core-collapse physics and obtaining the remnant black hole distribution as a derived by-product. In doing so, we constrain the stellar physics necessary to explain the astrophysical distribution of black hole properties under a given model. We apply this to the mapping between initial mass and remnant black hole mass, accounting for mass-dependent mass loss using a simple parameterized description. Allowing the parameters of the initial mass–remnant mass relationship to evolve with redshift permits correlated and physically reasonable changes to features in the mass function. We find that the current data are consistent with no redshift evolution in the core–remnant mass relationship, but place only weak constraints on the change of these parameters. This procedure can be applied to modeling any physical process underlying the astrophysical distribution. We illustrate this by applying our model to the pulsational pair instability supernova (PPISN) process, previously proposed as an explanation for the observed excess of black holes at ∼35M_⊙. Placing constraints on the reaction rates necessary to explain the PPISN parameters, we concur with previous results in the literature that the peak observed at ∼35M_⊙is unlikely to be a signature from the PPISN process as presently understood.