Magnetically Driven Accretion Flows in the Kerr Metric I: Models and Overall Structure

This is the first in a series of papers that investigate the properties of accretion flows in the Kerr metric through three-dimensional, general relativistic magnetohydrodynamic simulations of tori with a near-Keplerian initial angular velocity profile. We study four models with increasing black hole spin, from a/M=0 to 0.998, for which the structural parameters of the initial tori are maintained nearly constant. The subsequent accretion flows arise self-consistently from stresses and turbulence created by the magnetorotational instability. We investigate the overall evolution and the late-time global structure in the resulting non-radiative accretion flows, including the magnetic fields within the disks, the properties of the flow in the plunging region, and the flux of conserved quantities into the black hole. Independent of black hole spin, the global structure is described in terms of five regions: the main disk body, the coronal envelope, the inner disk, consisting of an inner torus and plunging region, an evacuated axial funnel, and a bi-conical outflow confined to the corona-funnel boundary. We find evidence for lower accretion rates, stronger funnel-wall outflows, and increased stress in the near hole region with increasing black hole spin.Comment: 29 pages, 15 figures, version of paper with high-resolution figures and links to animations available at http://www.astro.virginia.edu/~jd5v/KD_movies.ht

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

Synchrotron Radiation From Radiatively Inefficient Accretion Flow Simulations: Applications to Sgr A*

We calculate synchrotron radiation in three-dimensional pseudo-Newtonian magnetohydrodynamic simulations of radiatively inefficient accretion flows. We show that the emission is highly variable at optically thin frequencies, with order of magnitude variability on time-scales as short as the orbital period near the last stable orbit; this emission is linearly polarized at the 20-50 % level due to the coherent toroidal magnetic field in the flow. At optically thick frequencies, both the variability amplitude and polarization fraction decrease significantly with decreasing photon frequency. We argue that these results are broadly consistent with the observed properties of Sgr A* at the Galactic Center, including the rapid infrared flaring.Comment: Accepted for publication in Ap

Magnetically Driven Accretion in the Kerr Metric III: Unbound Outflows

We have carried out fully relativistic numerical simulations of accretion disks in the Kerr metric. In this paper we focus on the unbound outflows that emerge self-consistently from the accretion flow. These outflows are found in the axial funnel region and consist of two components: a hot, fast, tenuous outflow in the axial funnel proper, and a colder, slower, denser jet along the funnel wall. Although a rotating black hole is not required to produce these unbound outflows, their strength is enhanced by black hole spin. The funnel-wall jet is excluded from the axial funnel due to elevated angular momentum, and is also pressure-confined by a magnetized corona. The tenuous funnel outflow accounts for a significant fraction of the energy transported to large distances in the higher-spin simulations. We compare the outflows observed in our simulations with those seen in other simulations.Comment: 33 pages, 8 figures, ApJ submitte

Magnetically Driven Accretion Flows in the Kerr Metric IV: Dynamical Properties of the Inner Disk

This paper continues the analysis of a set of general relativistic 3D MHD simulations of accreting tori in the Kerr metric with different black hole spins. We focus on bound matter inside the initial pressure maximum, where the time-averaged motion of gas is inward and an accretion disk forms. We use the flows of mass, angular momentum, and energy in order to understand dynamics in this region. The sharp reduction in accretion rate with increasing black hole spin reported in Paper I of this series is explained by a strongly spin-dependent outward flux of angular momentum conveyed electromagnetically; when a/M > 0.9, this flux can be comparable to the inward angular momentum flux carried by the matter. In all cases, there is outward electromagnetic angular momentum flux throughout the flow; in other words, contrary to the assertions of traditional accretion disk theory, there is in general no "stress edge", no surface within which the stress is zero. The retardation of accretion in the inner disk by electromagnetic torques also alters the radial distribution of surface density, an effect that may have consequences for observable properties such as Compton reflection. The net accreted angular momentum is sufficiently depressed by electromagnetic effects that in the most rapidly-spinning black holes mass growth can lead to spindown. Spinning black holes also lose energy by Poynting flux; this rate is also a strongly increasing function of black hole spin, rising to 10% or more of the rest-mass accretion rate at very high spin. As the black hole spins faster, the path of the Poynting flux changes from being predominantly within the accretion disk to predominantly within the funnel outflow.Comment: 38 pages, submitted to Ap

Magnetically Driven Accretion Flows in the Kerr Metric II: Structure of the Magnetic Field

We present a detailed analysis of the magnetic field structure found in general relativistic 3D MHD simulations of accreting tori in the Kerr metric with different black hole spins. Among the properties analyzed are the field strength as a function of position and black hole spin, the shapes of field lines, the degree to which they connect different regions, and their degree of tangling. We investigate prior speculations about the structure of the magnetic fields and discuss how frequently certain configurations are seen in the simulations. We also analyze the distribution of current density, with a view toward identifying possible locations for magnetic energy dissipation.Comment: Submitted to ApJ. PDF and PostScript files with high-resolution figures are available at http://www.pha.jhu.edu/~shirose/GRMHD/PaperII

A General Relativistic Magnetohydrodynamics Simulation of Jet Formation

We have performed a fully three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion disk around a Schwarzschild black hole with a free-falling corona. The initial simulation results show that a bipolar jet (velocity $\sim 0.3c$) is created as shown by previous two-dimensional axisymmetric simulations with mirror symmetry at the equator. The 3-D simulation ran over one hundred light-crossing time units ($\tau_{\rm S} = r_{\rm S}/c$ where $r_{\rm S} \equiv 2GM/c^2$) which is considerably longer than the previous simulations. We show that the jet is initially formed as predicted due in part to magnetic pressure from the twisting the initially uniform magnetic field and from gas pressure associated with shock formation in the region around $r = 3 r_{\rm S}$. At later times, the accretion disk becomes thick and the jet fades resulting in a wind that is ejected from the surface of the thickened (torus-like) disk. It should be noted that no streaming matter from a donor is included at the outer boundary in the simulation (an isolated black hole not binary black hole). The wind flows outwards with a wider angle than the initial jet. The widening of the jet is consistent with the outward moving torsional Alfv\'{e}n waves (TAWs). This evolution of disk-jet coupling suggests that the jet fades with a thickened accretion disk due to the lack of streaming material from an accompanying star.Comment: 27 pages, 8 figures, revised and accepted to ApJ (figures with better resolution: http://gammaray.nsstc.nasa.gov/~nishikawa/schb1.pdf

Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows

We present three-dimensional MHD simulations of rotating radiatively inefficient accretion flows onto black holes. In the simulations, we continuously inject magnetized matter into the computational domain near the outer boundary, and we run the calculations long enough for the resulting accretion flow to reach a quasi-steady state. We have studied two limiting cases for the geometry of the injected magnetic field: pure toroidal field and pure poloidal field. In the case of toroidal field injection, the accreting matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion disk. The disk resembles in many respects the convection-dominated accretion flows found in previous numerical and analytical investigations of viscous hydrodynamic flows. Models with poloidal field injection evolve through two distinct phases. In an initial transient phase, the flow forms a relatively flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow. However, when the flow later achieves steady state, it changes in character completely. The magnetized accreting gas becomes two-phase, with most of the volume being dominated by a strong dipolar magnetic field from which a thermal low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating radial streams which `diffuse' through the magnetic field with the help of magnetic reconnection events.Comment: 35 pages including 3 built-in plots and 14 separate jpg-plots; version accepted by Ap

Pair Production in Low Luminosity Galactic Nuclei

Electron-positron pairs may be produced near accreting black holes by a variety of physical processes, and the resulting pair plasma may be accelerated and collimated into a relativistic jet. Here we use a self-consistent dynamical and radiative model to investigate pair production by \gamma\gamma collisions in weakly radiative accretion flows around a black hole of mass M and accretion rate \dot{M}. Our flow model is drawn from general relativistic magnetohydrodynamic simulations, and our radiation field is computed by a Monte Carlo transport scheme assuming the electron distribution function is thermal. We argue that the pair production rate scales as r^{-6} M^{-1} \dot{M}^{6}. We confirm this numerically and calibrate the scaling relation. This relation is self-consistent in a wedge in M, \dot{M} parameter space. If \dot{M} is too low the implied pair density over the poles of the black hole is below the Goldreich-Julian density and \gamma\gamma pair production is relatively unimportant; if \dot{M} is too high the models are radiatively efficient. We also argue that for a power-law spectrum the pair production rate should scale with the observables L_X \equiv X-ray luminosity and M as L_X^2 M^{-4}. We confirm this numerically and argue that this relation likely holds even for radiatively efficient flows. The pair production rates are sensitive to black hole spin and to the ion-electron temperature ratio which are fixed in this exploratory calculation. We finish with a brief discussion of the implications for Sgr A* and M87.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in Ap