The Astrophysical Journal
2026
Shared — curated by The Gufo editorial team
Revising the Spin and Kick Connection in Isolated Binary Black Holes
Vishal Baibhav*, Vicky Kalogera
Columbia University · Northwestern University
Received: 2026-10-22 · Published: 2026-10-01
© 2026 The Author(s) and the original publisher.
Abstract
Abstract The origin of black hole (BH) spins remains one of the least understood aspects of BHs. Despite many uncertainties, it is commonly assumed that if BHs originated from isolated massive star binaries, their spins should be aligned with the orbital angular momentum of the binary system. This assumption stems from the notion that BHs inherit their spins from their progenitor stars. In this study, we relax this long-held viewpoint and explore various mechanisms that can spin up BHs before or during their formation. In addition to natal spins, we discuss physical processes that can spin BHs isotropically, parallel to natal kicks, and perpendicular to natal kicks. These different mechanisms leave behind distinct imprints on the observable distributions of spin magnitudes, spin–orbit misalignments, and the effective inspiral spin of merging binaries. In particular, these mechanisms allow even the binaries originating in the field to exhibit precession and retrograde spin ( χ eff < 0). This broadens the parameter space allowed for isolated binary evolution into regimes which were previously thought to be exclusive to dynamically assembled binaries.
Summary
Black hole spins are one of the most puzzling properties of these objects, and much of gravitational-wave astrophysics assumes that if black holes form in isolated binary star systems, their spins should line up with the orbit's angular momentum because the black hole simply inherits the spin of its progenitor star. This paper challenges that assumption by exploring additional physical mechanisms that could tilt or realign black hole spins at the moment of formation, rather than treating natal spin as the only ingredient.
The authors consider spin-up processes that act isotropically, as well as spin imparted parallel or perpendicular to the natal kick a black hole receives when it forms. They show that each of these mechanisms leaves a distinctive fingerprint on observable quantities such as spin magnitude distributions, the degree of misalignment between spin and orbital angular momentum, and the effective inspiral spin parameter measured in gravitational-wave events. Crucially, these mechanisms mean that even binaries formed through the standard isolated-binary channel can display spin precession and even negative effective spins, outcomes usually attributed only to dynamically assembled binaries in dense stellar environments.
Plain-language summary by The Gufo; the paper is the authors’ own text.
Key findings
- Black hole spin need not be purely inherited from the progenitor star and aligned with the orbital angular momentum, as commonly assumed for isolated binaries.
- Several distinct mechanisms, spin imparted isotropically, parallel to the natal kick, or perpendicular to the natal kick, can misalign black hole spins at birth.
- These different spin-up mechanisms produce distinguishable signatures in the predicted distributions of spin magnitude, spin-orbit misalignment, and effective inspiral spin.
- Isolated binary black holes can thus exhibit spin precession and even negative effective inspiral spins, features previously thought to indicate dynamical formation channels.
