Simulations of Magnetized Disks Around Black Holes: Effects of Black Hole Spin, Disk Thickness, and Magnetic Field Geometry

TL;DR

This study uses 3D general relativistic magnetohydrodynamic simulations to evaluate the Novikov-Thorne model's accuracy for magnetized accretion disks around black holes, considering effects of spin, thickness, and magnetic field structure.

Contribution

It provides the first detailed simulation-based assessment of the NT model's validity for realistic, magnetized, non-razor-thin accretion disks around black holes.

Findings

Deviations from NT model are small for thin disks at low luminosity.

Magnetic field geometry influences stress and luminosity inside the ISCO.

Thin disks in X-ray binaries are well-described by NT model at luminosities below 30% Eddington.

Abstract

The standard general relativistic model of a razor-thin accretion disk around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical accretion disks are not razor-thin, it is uncertain whether the shear stress necessarily vanishes at the ISCO, and the magnetic field, which is thought to drive turbulence in disks, may contain large-scale structures that do not behave like a simple local scalar viscosity. We describe three-dimensional general relativistic magnetohydrodynamic simulations of accretion disks around black holes with a range of spin parameters, and we use the simulations to assess the validity of the NT model. Our fiducial initial magnetic field consists of multiple (alternating polarity) poloidal field loops whose shape is roughly isotropic in the disk in order to match the isotropic turbulence expected in the poloidal plane. For a thin disk with an aspect ratio |h/r| ~ 0.07 around a non-spinning black hole, we find a decrease in the accreted specific angular momentum of 2.9% relative to the NT model and an excess luminosity from inside the ISCO of 3.5%. The deviations in the case of spinning black holes are of the same order. In addition, the deviations decrease with decreasing |h/r|. We therefore conclude that magnetized thin accretion disks in x-ray binaries in the thermal/high-soft spectral state ought to be well-described by the NT model, especially at luminosities below 30% of Eddington where we expect a very small disk thickness |h/r| <~ 0.05. We also discuss how the stress and the luminosity inside the ISCO depend on the assumed initial magnetic field geometry. (abridged)