Secular Spin-Orbit Resonances of Black Hole Binaries in AGN Disks

The spin-orbit misalignment of stellar-mass black hole (sBH) binaries provides important constraints on the formation channels of merging sBHs. Here, we study the role of secular spin-orbit resonance in the evolution of an sBH binary component around a supermassive BH (SMBH) in an AGN disk. We consider the sBH's spin precession due to the J (2) moment introduced by a circum-sBH disk within the warping/breaking radius of the disk. We find that the sBH's spin-orbit misalignment (obliquity) can be excited via spin-orbit resonance between the sBH binary's orbital nodal precession and the sBH spin precession driven by a massive circum-sBH disk. Using an alpha-disk model with Bondi-Hoyle-Lyttleton accretion, the resonances typically occur for sBH binaries with semimajor axis of 1 au and at a distance of similar to 1000 au around a 10(7 ) M (circle dot) SMBH. The spin-orbit resonances can lead to high sBH obliquities and a broad distribution of sBH binary spin-spin misalignments. However, we note that the Bondi-Hoyle-Lyttleton accretion is much higher than that of Eddington accretion, which typically results in spin precession being too low to trigger spin-orbit resonances. Thus, secular spin-orbit resonances can be quite rare for sBHs in AGN disks.

Automated summary

This study investigates the impact of secular spin-orbit resonance on the evolution of stellar-mass black hole binaries around a supermassive black hole. It is found that spin-orbit resonance can lead to misalignment between the spins and orbits of the black holes, but the occurrence of such resonances is quite rare in AGN disks.