Perturbing AGN Accretion Disks with Stars and Moderately Massive Black Holes: Implications for Changing-Look AGN and Quasi-Periodic Eruptions

TL;DR

This paper proposes a model where orbiting companions, such as stars or black holes, interact with AGN accretion disks to explain extreme variability phenomena like changing-look AGN and quasi-periodic eruptions, emphasizing the role of eccentric orbits and companion mass.

Contribution

The study introduces a novel model linking orbiting companions to AGN variability, highlighting the importance of companion mass and orbital eccentricity in explaining observed phenomena.

Findings

Lighter SMBH companions are needed for changing-look AGN.

Highly eccentric orbits are essential for non-repeating CL AGN.

Stars with eccentric orbits can produce QPE-like accretion flares.

Abstract

We present a model that explains some extreme variability phenomena observed in active galactic nuclei (AGN). In this model, an orbiting companion interacts with the accretion disk surrounding the central supermassive black hole (SMBH). This interaction excites spiral density waves, leading to bursts of mass inflow lasting a few orbital timescales, whose mass content depends sensitively on the sphere of influence of the orbiting companion. To explain changing-look (CL) AGN, we find that lighter SMBH companions are necessary, while we generally exclude stellar companions. The moderately massive black hole perturber must be on a highly eccentric orbit in order to account for the non-repeating nature of most CL AGN. When applied to quasi-periodic eruptions (QPEs), we find that stars with highly eccentric orbits and close pericenter passages can produce accretion flares with QPE-like characteristics. For many QPEs, low-mass main-sequence stars or stripped-envelope stars are required. Although moderately massive black hole perturbers could also match the observed properties of QPEs, the gravitational-wave merger timescales of such binary systems are prohibitively short.