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

Understand

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.