We present a detailed three dimensional magnetohydrodynamic (MHD)
simulation describing the inner region of a disk accreting onto a black
hole. To avoid the technical complications of general relativity, the
dynamics are treated in Newtonian fashion using the pseudo-Newtonian
Paczýnski-Wiita potential. The disk evolves due to angular
momentum transport which is produced naturally from MHD turbulence
generated by the magnetorotational instability. We find that the
resulting stress is continuous across the marginally stable orbit, in
contradiction with the widely-held assumption that the stress should go
to zero there. As a consequence, the specific angular momentum of the
matter accreted into the hole is smaller than the specific angular
momentum at the marginally stable orbit. The disk exhibits large
fluctuations in almost every quantity, both spatially and temporally.
In particular, the ratio of stress to pressure (the local analog of the
Shakura-Sunyaev
parameter)
exhibits both systematic gradients and large fluctuations; from
10-2 in the disk midplane at large radius, it rises to 10
both at a few gas density scaleheights above the plane at large radius,
and near the midplane well inside the plunging region. Driven in part
by large-amplitude waves excited near the marginally stable orbit, both
the mass accretion rate and the integrated stress exhibit large
fluctuations whose Fourier power spectra are smooth ``red" power-laws
stretching over several orders of magnitude in timescale.
Subject headings: accretion, accretion disks, MHD, black holes
MPEG movies of this paper's simulation:
- 3D simulation: Side View 1.2 MBytes
- 3D simulation: View from above 1.2 MBytes
- View with Marginally Stable Orbit 2.7 MBytes
- Equatorial plane 3.1 MBytes
Download a gzipped postscript file of the latest version of the paper (revised 8/30/00):