Spin flip

A black hole spin-flip occurs when the spin axis of a rotating black hole undergoes a sudden change in orientation due to absorption of a second (smaller) black hole. Spin-flips are believed to be a consequence of galaxy mergers, when two supermassive black holes form a bound pair at the center of the merged galaxy and coalesce after emitting gravitational waves. Spin-flips are significant astrophysically since a number of physical processes are associated with black hole spins; for instance, jets in active galaxies are believed to be launched parallel to the spin axes of supermassive black holes. A change in the rotation axis of a black hole due to a spin-flip would therefore result in a change in the direction of the jet.

A spin-flip is a late stage in the evolution of a binary black hole. The binary consists of two black holes, with masses ${\displaystyle M_{1}}$ and ${\displaystyle M_{2}}$, that revolve around their common center of mass. The total angular momentum ${\displaystyle J}$ of the binary system is the sum of the angular momentum of the orbit, ${\displaystyle {L}}$, plus the spin angular momenta ${\displaystyle {S}_{1,2}={S}_{1}+{S}_{2}}$ of the two holes. If we write ${\displaystyle \mathbf {M_{1}} ,\mathbf {M_{2}} }$ as the masses of each hole and ${\displaystyle \mathbf {a_{1}} ,\mathbf {a_{2}} }$ as their Kerr parameters, then use the angle from north of their spin axes as given by ${\displaystyle \theta }$, we can write,

...
Wikipedia