2025/10/30 by Jiao, Cheng-Liang, Zhu, Liying, Zhao, Er-gang +1
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2510.26111
Stellar-mass compact objects (COs) embedded in active galactic nucleus (AGN) discs are commonly assumed to accrete via Bondi or Bondi-Hoyle-Lyttleton (BHL) prescriptions, neglecting gas angular momentum. We show that differential rotation in AGN discs can impart non-negligible angular momentum, in which case accretion proceeds through a viscous disc rather than Bondi/BHL flow. Our model provides a new framework estimating the CO accretion rate as MCO = min\Mvis, MBHL\, where the viscous rate Mvis accounts for gas--CO relative motion decomposed into a local gradient term (due to differential rotation) and bulk motion (from differing orbital parameters). This rate can be expressed as Mvis = αξ(rH/rBHL)3MBHL, where ξ is a coefficient of order unity. It can also be approximately scaled to the global AGN accretion rate as Mvis ∝ M1, with the scaling coefficients in both forms determined by the specific dynamical configuration. The accretion is viscosity-limited when q > [αξ(1+M2)3/3]1/2 h3, where q is the mass ratio between the CO and the supermassive black hole, α the viscosity parameter, M the Mach number of the bulk relative motion, and h the aspect ratio of the AGN disc. In thin AGN discs this condition is satisfied for most stellar-mass or more massive COs. Our framework also naturally allows for the inclusion of established outflow corrections, thereby enabling a more realistic treatment of super-Eddington flows. Our formulation thus improves upon Bondi/BHL prescriptions and offers a more physically motivated basis for studying CO evolution in AGN environments.