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

TL;DR

This paper proposes that close binaries of supermassive black holes with high eccentricities can trigger changing-look AGN phenomena within a single orbit by modulating accretion processes through tidal interactions.

Contribution

It introduces a novel model where binary black holes induce changing-look transitions via tidal interactions and mini-disk dynamics, explaining rapid AGN variability.

Findings

Binary black holes can trigger CL-AGN transitions within one orbit.

Tidal interactions cause rapid accretion changes and flares.

Mini-disk rotation direction influences AGN type transitions.

Abstract

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.