The X-shaped galaxies have received much attention following the suggestion in 2002 that they might be the sites of spin-flips associated with the recent coalescence of two supermassive black holes.
Unlike the classical FRII sources, the X-shaped galaxies exhibit two, misaligned pairs of radio lobes of comparable extent.
In their original catalog of 11 X-shaped galaxies, Leahy and Parma[3] proposed that the "wings were created in an earlier outburst, some tens of Myrs previous to the current renewal of nuclear activity, during which time the ejection axis has precessed."
Since the lobes are produced by jets that are launched perpendicularly to the inner accretion disk, and since the accretion disk is constrained by the Bardeen-Petterson effect to lie perpendicular to the black hole's spin axis, a change in the spin orientation implies a change in the direction of the lobes.
Alternative models to explain the X-shaped sources include a warping instability of the accretion disk;[5] backflow of gas along the active lobes[6] and binary-disk interactions before coalescence.