Changing-look active galactic nuclei: close binaries of supermassive
  black holes in action
  Changing-look active galactic nuclei (CL-AGNs) as a new subpopulation
challenge some fundamental physics of AGNs because the timescales of the
phenomenon can hardly be reconciled with accretion disk models. In this
Letter{\textit{}}, we demonstrate the extreme case: close binaries of
supermassive black holes (CB-SMBHs) with high eccentricities are able to
trigger the CL transition through one orbit. In this scenario, binary black
holes build up their own mini-disks by peeling gas off the inner edges of the
circumbinary disk during the apastron phase, after which they tidally interact
with the disks during the periastron phase to efficiently exchange angular
momentum within one orbital period. For mini-disks rotating retrograde to the
orbit, the tidal torque rapidly squeezes the tidal parts of the mini-disks into
a much smaller radius, which rapidly results in higher accretion and short
flares before the disks decline into type-2 AGNs. Prograde-rotation mini-disks
gain angular momentum from the binary and rotate outward, which causes a rapid
turn-off from type-1 to type-2. Turn-on occurs around the apastron phase.
CB-SMBHs control cycle transitions between type-1 and type-2 with orbital
periods but allow diverse properties in CL-AGN light curves.
