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 M 1 {displaystyle M_{1}} and M 2 {displaystyle M_{2}} , that revolve around their common center of mass. The total angular momentum J {displaystyle J} of the binary system is the sum of the angular momentum of the orbit, L {displaystyle {L}} , plus the spin angular momenta S 1 , 2 = S 1 + S 2 {displaystyle {S}_{1,2}={S}_{1}+{S}_{2}} of the two holes. If we write M 1 , M 2 {displaystyle mathbf {M_{1}} ,mathbf {M_{2}} } as the masses of each hole and a 1 , a 2 {displaystyle mathbf {a_{1}} ,mathbf {a_{2}} } as their Kerr parameters, then use the angle from north of their spin axes as given by θ {displaystyle heta } , we can write,Black hole spin-flips were first discussed in the context of a particular class of radio galaxy, the X-shaped radio sources. The X-shaped galaxies exhibit two, misaligned pairs of radio lobes: the 'active' lobes and the 'wings'. It is believed that the wings are oriented in the direction of the jet prior to the spin-flip, and that the active lobes point in the current jet direction. The spin-flip could have been caused by absorption of a second black hole during a galaxy merger.