Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori

This paper presents an initial survey of the properties of accretion flows in the Kerr metric from three-dimensional, general relativistic magnetohydrodynamic simulations of accretion tori. We consider three fiducial models of tori around rotating, both prograde and retrograde, and nonrotating black holes; these three fiducial models are also contrasted with axisymmetric simulations and a pseudo-Newtonian simulation with equivalent initial conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and high-resolution version of figures available at http://www.astro.virginia.edu/~jd5

Global MHD Simulations of Cylindrical Keplerian Disks

This paper presents a series of global three dimensional accretion disk simulations carried out in the cylindrical limit in which the vertical component of the gravitational field is neglected. The simulations use a cylindrical pseudo-Newtonian potential to model the main dynamical properties of the Schwarzschild metric. The disks are initially constant density with a Keplerian angular momentum distribution and contain a weak toroidal or vertical field. These simulations reaffirm many of the conclusions of previous local simulations. The magnetorotational instability grows rapidly and produces MHD turbulence with a significant Maxwell stress which drives accretion. Tightly-wrapped low-$m$ spiral waves are prominent. In some simulations radial variations in Maxwell stress concentrate gas into rings, creating substantial spatial inhomogeneities. There is a nonzero stress at the marginally stable orbit which produces a small decline in specific angular momentum inside the last stable orbit. Detailed comparisons between simulations are used to examine the effects of computational domain and equation of state. Simulations that begin with vertical fields have greater field amplification and higher ratios of stress to magnetic pressure compared with those beginning with toroidal fields. In contrast to MHD, hydrodynamics alone neither creates nor sustains turbulence.Comment: Submitted to the Astrophysical Journal Web version of paper and MPEG animations can be found at http://www.astro.virginia.edu/~jh8h/cylinder

The Effect of Resistivity on the Nonlinear Stage of the Magnetorotational Instability in Accretion Disks

We present three-dimensional magnetohydrodynamic simulations of the nonlinear evolution of the magnetorotational instability (MRI) with a non-zero Ohmic resistivity. The properties of the saturated state depend on the initial magnetic field configuration. In simulations with an initial uniform vertical field, the MRI is able to support angular momentum transport even for large resistivities through the quasi-periodic generation of axisymmetric radial channel solutions rather than through the maintenance of anisotropic turbulence. Simulations with zero net flux show that the angular momentum transport and the amplitude of magnetic energy after saturation are significantly reduced by finite resistivity, even at levels where the linear modes are only slightly affected. This occurs at magnetic Reynolds numbers expected in low, cool states of dwarf novae, these results suggest that finite resistivity may account for the low and high angular momentum transport rates inferred for these systems.Comment: 8 figures, accepted for publication in Ap

What's the point of knowing how?

Why is it useful to talk and think about knowledge-how? Using Edward Craig’s discussion of the function of the concepts of knowledge and knowledge-how as a jumping off point, this paper argues that considering this question can offer us new angles on the debate about knowledge-how. We consider two candidate functions for the concept of knowledge-how: pooling capacities, and mutual reliance. Craig makes the case for pooling capacities, which connects knowledge-how to our need to pool practical capacities. I argue that the evidence is much more equivocal. My suggested diagnosis is that the concept of knowledge-how plays both functions, meaning that the concept of knowledge-how is inconsistent, and that the debate about knowledge-how is at least partly a metalinguistic negotiation. In closing, I suggest a way to revise the philosophical concept of knowledge how

Vortices in Thin, Compressible, Unmagnetized Disks

We consider the formation and evolution of vortices in a hydrodynamic shearing-sheet model. The evolution is done numerically using a version of the ZEUS code. Consistent with earlier results, an injected vorticity field evolves into a set of long-lived vortices, each of which has a radial extent comparable to the local scale height. But we also find that the resulting velocity field has a positive shear stress, . This effect appears only at high resolution. The transport, which decays with time as t^-1/2, arises primarily because the vortices drive compressive motions. This result suggests a possible mechanism for angular momentum transport in low-ionization disks, with two important caveats: a mechanism must be found to inject vorticity into the disk, and the vortices must not decay rapidly due to three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ electronic edition

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

Accretion of low angular momentum material onto black holes: 2D magnetohydrodynamical case

We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. We study accretion flows by means of numerical 2D, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus which is made of the material that has too much angular momentum to be accreted directly. The torus accretes, however, because of the transport of angular momentum due to the magnetorotational instability (MRI). Initially, accretion is dominated by the polar funnel, as in the hydrodynamic inviscid case, where material has zero or very low angular momentum. At the later phase of the evolution, the torus thickens towards the poles and develops a corona or an outflow or both. Consequently, the mass accretion through the funnel is stopped. The accretion of rotating gas through the torus is significantly reduced compared to the accretion of non-rotating gas (i.e., the Bondi rate). It is also much smaller than the accretion rate in the inviscid, weakly rotating case.Our results do not change if we switch on or off resistive heating. Overall our simulations are very similar to those presented by Stone, Pringle, Hawley and Balbus despite different initial and outer boundary conditions. Thus, we confirm that MRI is very robust and controls the nature of radiatively inefficient accretion flows.Comment: submitted in Ap

An Accretion-Jet Model for Black Hole Binaries: Interpreting the Spectral and Timing Features of XTE J1118+480

Multi-wavelength observations of the black hole X-ray binary XTE J1118+480 have offered abundant spectral and timing information about the source, and have thus provided serious challenges to theoretical models. We propose a coupled accretion-jet model to interpret the observations. We model the accretion flow as an outer standard thin accretion disk truncated at a transition radius by an inner hot accretion flow. The accretion flow accounts for the observed UV and X-ray emission, but it substantially under-predicts the radio and infrared fluxes, even after we allow for nonthermal electrons in the hot flow. We attribute the latter components to a jet. We model the jet emission by means of the internal shock scenario which is widely employed for gamma-ray bursts. In our accretion-jet model of XTE J1118+480, the jet dominates the radio and infrared emission, the thin disk dominates the UV emission, and the hot flow produces most of the X-ray emission. The optical emission has contributions from all three components: jet, thin disk, and hot flow. The model qualitatively accounts for timing features, such as the intriguing positive and negative time lags between the optical and X-ray emission, and the wavelength-dependent variability amplitude.Comment: 27 pages, 4 figures (one in color); to appear in ApJ in Feb. 200