Vishal Baibhav, Vicky Kalogera

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.