Stable mass transfer can explain massive binary black hole mergers with a high spin component

TL;DR

This paper proposes that stable Case A mass transfer during binary evolution can significantly spin up black holes, explaining the high spins observed in some massive binary black hole mergers.

Contribution

It introduces a new mechanism involving stable mass transfer to account for high black hole spins in binary mergers, supported by detailed evolutionary calculations.

Findings

Stable mass transfer can increase black hole spins.

High spins observed in some mergers can be explained by this process.

Mildly super-Eddington accretion is sufficient for spin-up.

Abstract

Recent gravitational wave observations showed that binary black hole (BBH) mergers with massive components are more likely to have high effective spins. In the model of isolated binary evolution, BH spins mainly originate from the angular momenta of the collapsing cores before BH formation. Both observations and theories indicate that BHs tend to possess relatively low spins, the origin of fast-spinning BHs remains a puzzle. We investigate an alternative process that stable Case A mass transfer may significantly increase BH spins during the evolution of massive BH binaries. We present detailed binary evolution calculations and find that this process can explain observed high spins of some massive BBH mergers under the assumption of mildly super-Eddington accretion.