Changing-look active galactic nuclei: close binaries of supermassive black holes in action

J. -M. Wang, E. Bon

Published 2020-10-09Version 1

Changing-look active galactic nuclei (CL-AGNs) as a new sub-population are challenging some fundamental physics of AGNs since timescales of the phenomenon hardly reconcile with accretion disk models. In this {\textit{Letter}}, we demonstrate the extreme case that 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 via peeling gas off inner edges of the circumbinary disk during the apastron phase after then they tidally interact with the disks during the periastron phase to efficiently exchange angular momentum (AM) within one orbital period. For mini-disks with rotation retrograde to the orbit, the tidal torque rapidly squeezes the tidal parts of the mini-disks into much smaller radius, resulting in an elevated accretion rapidly and short flares before declining into type-2 AGNs. In case of prograde mini-disks, they gain AM from the binary and rotate outward, giving rise to a rapid turn-off from type-1 to -2. Turn-on happens around the apastron phase. CB-SMBHs control cycle transitions between type-1 and -2 with orbital periods but allow diverse properties of light curves of CL-AGNs.