Turbulence in Global Simulations of Magnetized Thin Accretion Disks

We use a global magnetohydrodynamic simulation of a geometrically thin accretion disk to investigate the locality and detailed structure of turbulence driven by the magnetorotational instability (MRI). The model disk has an aspect ratio $H / R \simeq 0.07$, and is computed using a higher-order Godunov MHD scheme with accurate fluxes. We focus the analysis on late times after the system has lost direct memory of its initial magnetic flux state. The disk enters a saturated turbulent state in which the fastest growing modes of the MRI are well-resolved, with a relatively high efficiency of angular momentum transport $> \approx 2.5 \times 10^{-2}$. The accretion stress peaks at the disk midplane, above and below which exists a moderately magnetized corona with patches of superthermal field. By analyzing the spatial and temporal correlations of the turbulent fields, we find that the spatial structure of the magnetic and kinetic energy is moderately well-localized (with correlation lengths along the major axis of $2.5H$ and $1.5H$ respectively), and generally consistent with that expected from homogenous incompressible turbulence. The density field, conversely, exhibits both a longer correlation length and a long correlation time, results which we ascribe to the importance of spiral density waves within the flow. Consistent with prior results, we show that the mean local stress displays a well-defined correlation with the local vertical flux, and that this relation is apparently causal (in the sense of the flux stimulating the stress) during portions of a global dynamo cycle. We argue that the observed flux-stress relation supports dynamo models in which the structure of coronal magnetic fields plays a central role in determining the dynamics of thin-disk accretion.Comment: 24 pages and 25 figures. MNRAS in press. Version with high resolution figures available from http://jila.colorado.edu/~krb3u/Thin_Disk/thin_disk_turbulence.pd

Enhanced Angular Momentum Transport in Accretion Disks

The status of our current understanding of angular momentum transport in accretion disks is reviewed. The last decade has seen a dramatic increase both in the recognition of key physical processes and in our ability to carry through direct numerical simulations of turbulent flow. Magnetic fields have at once powerful and subtle influences on the behavior of (sufficiently) ionized gas, rendering them directly unstable to free energy gradients. Outwardly decreasing angular velocity profiles are unstable. The breakdown of Keplerian rotation into MHD turbulence may be studied in some numerical detail, and key transport coefficients may be evaluated. Chandra observations of the Galactic Center support the existence of low luminosity accretion, which may ultimately prove amenable to global three-dimensional numerical simulation.Comment: 43 pages, 2 figures, to appear v.43 A.R.A.A. October 200

Off-equatorial orbits in strong gravitational fields near compact objects

Near a black hole or an ultracompact star, motion of particles is governed by strong gravitational field. Electrically charged particles feel also electromagnetic force arising due to currents inside the star or plasma circling around. We study a possibility that the interplay between gravitational and electromagnetic action may allow for stable, energetically bound off-equatorial motion of charged particles. This would represent well-known generalized Stormer's 'halo' orbits, which have been discussed in connection with the motion of dust grains in planetary magnetospheres. We demonstrate that such orbits exist and can be astrophysically relevant when a compact star or a black hole is endowed with a dipole-type magnetic field. In the case of Kerr-Newman solution, numerical analysis shows that the mutually connected gravitational and electromagnetic fields do not allow existence of stable halo orbits above the outer horizon of black holes. Such orbits are either hidden under the inner black-hole horizon, or they require the presence of a naked singularity.Comment: 16 pages, 7 figures, accepted in Class. Quantum Grav. (2008

Accretion of helium and metal-rich gas onto neutron stars and black holes at high luminosities

Ultraluminous X-ray sources fed by Wolf-Rayet star winds and X-ray bursters in ultracompact binaries with He or C white dwarfs have accretion disks, whose properties may significantly differ from those of pure H alpha-accretion disks. We have therefore included the dependence on charge number Z and mean molecular weights mu_{e/I} into the Shakura and Sunyaev (1973) scaling relations for the key parameters of the disk. Furthermore, we also consider the case of the pseudo-Newtonian potential of Paczynsky and Wiita (1980). These scaling relations might become useful, e.g., when estimating the illumination efficiency of the external parts of the disk. We also address the changes in the structure of the boundary (spreading) layer on the surface of neutron stars, occurring in the case of H depleted accretion disks.Comment: 10 page

Post-Newtonian approach to black hole-fluid systems

This work devises a formalism to obtain the equations of motion for a black hole-fluid configuration. Our approach is based on a post-Newtonian expansion and adapted to scenarios where obtaining the relevant dynamics requires long time-scale evolutions. These systems are typically studied with Newtonian approaches, which have the advantage that larger time steps can be employed than in full general-relativistic simulations but have the downside that important physical effects are not accounted for. The formalism presented here provides a relatively straightforward way to incorporate those effects in existing implementations, up to 2.5 post-Newtonian order, with lower computational costs than fully relativistic simulations

Numerical investigation of plasma-driven superradiant instabilities

Photons propagating in a plasma acquire an effective mass \u3bc, which is given by the plasma frequency and which scales with the square root of the plasma density. As noted previously in the literature, for electron number densities ne ~ 10 123 cm 123 (such as those measured in the interstellar medium) the effective mass induced by the plasma is \u3bc ~ 10 1212 eV. This would cause superradiant instabilities for spinning black holes of a few tens of solar masses. An obvious problem with this picture is that densities in the vicinity of black holes are much higher than in the interstellar medium because of accretion, and possibly also pair production. We have conducted numerical simulations of the superradiant instability in spinning black holes surrounded by a plasma with density increasing closer to the black hole, in order to mimic the effect of accretion. While we confirm that superradiant instabilities appear for plasma densities that are sufficiently low near the black hole, we find that astrophysically realistic accretion disks are unlikely to trigger such instabilities