Revising the Spin and Kick Connection in Isolated Binary Black Holes

TL;DR

This paper challenges the traditional assumption that black hole spins in isolated binaries are aligned with their orbital angular momentum, exploring mechanisms that can produce misaligned and retrograde spins, thus broadening the understanding of black hole spin origins.

Contribution

It introduces new mechanisms for black hole spin evolution that allow for misalignment and retrograde spins, expanding the parameter space of isolated binary evolution.

Findings

Black holes can acquire isotropic or kick-aligned spins before or during formation.

Observable spin distributions can include precession and retrograde spins in field binaries.

Mechanisms can produce spin-orbit misalignments similar to dynamical formation scenarios.

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 ($\chi_{\rm eff}<0$). This broadens the parameter space allowed for isolated binary evolution into regimes which were previously thought to be exclusive to dynamically assembled binaries.