On rapid migration and accretion within disks around supermassive black holes

TL;DR

This paper explores how objects within accretion disks around supermassive black holes can migrate and accrete, potentially explaining observed AGN phenomena and influencing disk stability.

Contribution

It introduces the concept of nuclear cluster objects migrating within AGN disks, showing their potential to dominate accretion and explain X-ray excesses.

Findings

Runaway outward migration of NCOs can produce Seyfert/quasar luminosities.

Multiple NCOs can dominate disk accretion over viscosity.

NCO activity may prevent gravitational instability in disks.

Abstract

Galactic nuclei should contain a cluster of stars and compact objects in the vicinity of the central supermassive black hole due to stellar evolution, minor mergers and gravitational dynamical friction. By analogy with protoplanetary migration, nuclear cluster objects (NCOs) can migrate in the accretion disks that power active galactic nuclei by exchanging angular momentum with disk gas. Here we show that an individual NCO undergoing runaway outward migration comparable to Type III protoplanetary migration can generate an accretion rate corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in an AGN disk can dominate traditional viscous disk accretion and at large disk radii, ensemble NCO migration and accretion could provide sufficient heating to prevent the gravitational instability from consuming disk gas in star formation. The magnitude and energy of the X-ray soft excess observed at ~0.1-1keV in Seyfert AGN could be explained by a small population of ~10^{2}-10^{3} accreting stellar mass black holes or a few ULXs. NCO migration and accretion in AGN disks are therefore extremely important mechanisms to add to realistic models of AGN disks.