Magnetohydrodynamic Accretion Flows Research Articles

Low-angular-momentum General Relativistic Magnetohydrodynamic Accretion Flows around Rotating Black Holes with Shocks

We investigate the global structure of the general relativistic magnetohydrodynamic (GRMHD) accretion flows around Kerr black holes containing shock waves, where the disk is threaded by radial and toroidal magnetic fields. We self-consistently solve the GRMHD equations that govern the flow motion inside the disk and for the first time, to our knowledge, we obtain the shock-induced global GRMHD accretion solutions around weakly as well as rapidly rotating black holes for a set of fundamental flow parameters, such as energy ( E ), angular momentum ( L ), radial magnetic flux (Φ), and isorotation parameter (F). We show that shock properties—namely, the shock radius (r sh), compression ratio (R), and shock strength (Ψ)—strongly depend on E , L , Φ, and F. We observe that the shock in the GRMHD flow continues to exist for a wide range of the flow parameters, which allows us to identify the effective domain of the parameter space in the L–E plane where shock solutions are feasible. Moreover, we examine the modification of the shock parameter space and find that it shifts towards the lower-angular-momentum values with increasing Φ and black hole spin (a k). Finally, we compute the critical radial magnetic flux (Φcri) that admits shocks in GRMHD flows and ascertain that Φcri is higher (lower) for a black hole of spin a k = 0.99 (0.0) and vice versa.

Study of general relativistic magnetohydrodynamic accretion flow around black holes

ABSTRACT We present a novel approach to study the global structure of steady, axisymmetric, advective, magnetohydrodynamic (MHD) accretion flow around black holes in full general relativity (GR). Considering ideal MHD conditions and relativistic equation of state (REoS), we solve the governing equations to obtain all possible smooth global accretion solutions. We examine the dynamical and thermodynamical properties of accreting matter in terms of the flow parameters, namely energy (${\cal E}$), angular momentum (${\cal L}$), and local magnetic fields. For a vertically integrated GRMHD flow, we observe that toroidal component (bϕ) of the magnetic fields generally dominates over radial component (br) at the disc equatorial plane. This evidently suggests that toroidal magnetic field indeed plays important role in regulating the disc dynamics. We further notice that the disc remains mostly gas pressure (pgas) dominated (β = pgas/pmag > 1, pmag refers magnetic pressure) except at the near horizon region, where magnetic fields become indispensable (β ∼ 1). We observe that Maxwell stress is developed that eventually yields angular momentum transport inside the disc. Towards this, we calculate the viscosity parameter (α) that appears to be radially varying. In addition, we examine the underlying scaling relation between α and β, which clearly distinguishes two domains coexisted along the radial extent of the disc. Finally, we discuss the utility of the present formalism in the realm of GRMHD simulation studies.

BLACK HOLE AURORA POWERED BY A ROTATING BLACK HOLE

We present a model for high-energy emission sources generated by a standing magnetohydrodynamical (MHD) shock in a black hole magnetosphere. The black hole magnetosphere would be constructed around a black hole with an accretion disk, where a global magnetic field could be originated by currents in the accretion disk and its corona. Such a black hole magnetosphere may be considered as a model for the central engine of active galactic nuclei, some compact X-ray sources and gamma-ray bursts. The energy sources of the emission from the magnetosphere are the gravitational and electromagnetic energies of magnetized accreting matters and the rotational energy of a rotating black hole. When the MHD shock generates in MHD accretion flows onto the black hole, the plasma's kinetic energy and hole's rotational energy can convert to radiative energy. In this letter, we demonstrate the huge energy output at the shock front by showing negative energy postshock accreting MHD flows for a rapidly rotating black hole. This means that the extracted energy from the black hole can convert to the radiative energy at the MHD shock front. When axisymmetric shock front is formed, we expect a ring-shaped region with very hot plasma near the black hole; the look would be like an "aurora". The high energy radiation generated from there would carry to us the information for the curved spacetime due to the strong gravity.

Three-dimensional radiative properties of hot accretion flows on to the Galactic Centre black hole

By solving radiative transfer equations, we examine three-dimensional radiative properties of a magnetohydrodynamic accretion flow model confronting with the observed spectrum of Sgr A*, in the vicinity of supermassive black hole at the Galactic centre. As a result, we find that the core of radio emission is larger than the size of the event horizon shadow and its peak location is shifted from the gravitational centre. We also find that the self-absorbed synchrotron emissions by the superposition of thermal electrons within a few tens of the Schwartzschild radius can account for low-frequency spectra below the critical frequency $\nu_{c}\approx 10^{12}$ Hz. Above the critical frequency, the synchrotron self-Compton emission by thermal electrons can account for variable emissions in recent near-infrared observations. In contrast to the previous study by Ohsuga et al. (2005), we found that the X-ray spectra by Bremsstrahlung emission of thermal electrons for the different mass accretion rates can be consistent with both the flaring state and the quiescent state of Sgr A* observed by {\it Chandra}.

X-Ray Emissions from Three-Dimensional Magnetohydrodynamic Coronal Accretion Flows

Abstract We calculated the radiation spectrum and its time variability of a black-hole accretion disk-corona system based on a three-dimensional magnetohydrodynamic (MHD) simulation. In explaining the spectral properties of active galactic nuclei, it is often assumed that they consist of a geometrically thin, optically thick disk and hot, optically thin corona surrounding the thin disk. As for a model of the corona, we adopted the simulation data of three-dimensional, non-radiative MHD accretion flows calculated by Y. Kato and coworkers, while for a thin disk we assumed a standard-type disk. We performed Monte-Carlo radiative transfer simulations of the corona, while taking into account the Compton scattering of soft photons from the thin disk by hot thermal electrons and coronal irradiation heating of the thin disk, which emits blackbody radiation. By adjusting the density parameter of the MHD coronal flow, we could produce emergent spectra that are consistent with those of typical Seyfert galaxies. Moreover, we found rapid time variability in the X-ray emission spectra, originating from a density fluctuation produced by a magnetorotational instability in the MHD corona. The features of reflection component including iron fluorescent line emission are also briefly discussed.

Parameter space study of the magnetohydrodynamic accretion flows around compact objects

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around compact objects and found all possible classes of solutions for non-relativistic adiabatic accretion flows. We divide the parameter space in terms of these classes. We study the possibility of the formation of the MHD shock waves and show how the strength of the shocks depends on the flow parameters. We also show regions of the parameter space where the shock conditions are not satisfied and therefore the shocks may oscillate. These solutions are astrophysically interesting as they could give rise to quasi-periodic oscillations seen in hard X-rays.

Gravitational Collapse of Magnetized Clouds. I. Ideal Magnetohydrodynamic Accretion Flow

We study the self-similar collapse of an isothermal magnetized rotating cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance from the accreting protostar, we find an analytic solution that corresponds to free fall onto a central mass point. The density distribution is not spherically symmetric but depends on the mass loading of magnetic field lines, which can be obtained by matching our inner solution to an outer collapse solution previously computed by Allen et al. The concentration of magnetic field trapped by the central mass point under field freezing, independent on the details of the starting state, creates a split-monopole configuration in which the magnetic field strength increases as the inverse square of the distance from the center. Under such conditions, the inflow eventually becomes sub-Alfvenic and the outward transfer of angular momentum by magnetic braking very efficient, thus preventing the formation of a centrifugally supported disk. Instead, the azimuthal velocity of the infalling gas decreases to zero at the center, and the gas spirals into the star. Therefore, the dissipation of dynamically important levels of magnetic field is a fundamental requisite for the formation of protoplanetary disks around young stars.

Standing Shocks in Transmagnetosonic Accretion Flows onto a Black Hole

Fast and slow magnetosonic shock formation is presented for stationary and axisymmetric magnetohydrodynamic (MHD) accretion flows onto a black hole. The shocked black hole accretion solution must pass through magnetosonic points at some locations outside and inside the shock location. We analyze critical conditions at the magnetosonic points and the shock conditions. Then, we show the restrictions on the flow parameters for strong shocks. We also show that a very hot shocked plasma is obtained for a very high energy inflow with a small number density. Such a MHD shock can appear very close to the event horizon and can be expected as a source of high-energy emissions. Examples of shocked MHD accretion flows are presented in the Schwarzschild case.

Spectral Properties of Three‐dimensional Magnetohydrodynamic Accretion Flows

In spite of a large number of global three-dimensional MHD simulations of accretion flows and jets being made recently, their astrophysical relevance for realistic situations is not well known. In order to examine to what extent the simulated MHD flows can account for the observed SED of Sgr A*, for the first time we calculate the emergent spectra from three-dimensional MHD flows in a wide range of wavelengths (from radio to X-ray) by solving the three-dimensional radiative transfer equations. We use the simulation data by Kato and coworkers and perform Monte Carlo radiative transfer simulations, in which synchrotron emission/absorption, free-free emission/absorption, and Compton/inverse Compton scattering are taken into account. We assume two-temperature plasmas and calculate electron temperatures by solving the electron energy equation. Only thermal electrons are considered. It is found that the three-dimensional MHD flow generally overproduces X-rays by means of bremsstrahlung radiation from the regions at large radii. A flatter density profile, ρ ∝ r-a with a < 1, than that of the ADAF, ρ ∝ r-3/2, is the main reason for this. If we restrict the size of the emission region to be as small as ~10rS, where rS is the Schwarzschild radius, the MHD model can reproduce the basic features of the observed SED of Sgr A* during its flaring state. Yet, the spectrum in the quiescent state remains to be understood. We discuss how to resolve this issue in the context of MHD flow models. Possibilities include modifications of the MHD flow structure by the inclusion of radiative cooling and/or significant contributions by nonthermal electrons. It is also possible that the present spectral results may be influenced by particular initial conditions. We also calculate the time-dependent spectral changes, finding that the fluxes fluctuate in a wide range of the frequency and the flux at each wavelength does not always vary coherently.

2:3 Twin Quasi-Periodic Oscillations in Magnetohydrodynamic Accretion Flows

We have studied the radial and vertical oscillations in three-dimensional magnetohydrodynamic (MHD) accretion flows around black holes. General-relativistic effects were taken into account by using a pseudo-Newtonian potential. We found that the structure of MHD flows is changed at $3.8 \,r_{\mathrm{s}} \leq r \leq 6.3 \,r_{\mathrm{s}}$, and that two pairs of quasi-periodic oscillations (twin QPOs) are excited in that region. The time evolution of the power spectrum density (PSD) indicates that these twin QPOs are most likely to be produced by a resonance between the Keplerian frequency, $\nu_\mathrm{K}$, and the epicyclic frequency, $\nu_{\kappa}$. The PSD shows that the lower peak frequency, $\nu_\mathrm{l}$, corresponds to, $\nu_\mathrm{K}$, while the upper peak frequency, $\nu_\mathrm{u}$, corresponds to $\nu_\mathrm{K} + \nu_{\kappa}$. The ratio of two peak frequencies is close to $2:3$. The results provide the first direct evidence for the excitation of resonant disk oscillation in MHD accretion flows.

Magnetohydrodynamic Accretion Flows: Formation of Magnetic Tower Jet and Subsequent Quasi–Steady State

We present three-dimensional magnetohydrodynamic (MHD) simulations of radiatively inefficient accretion flow around black holes. General relativistic effects are simulated by using the pseudo-Newtonian potential. We start calculations with a rotating torus threaded by localized poloidal magnetic fields with plasma-β, a ratio of the gas pressure to the magnetic pressure, β = 10 and 100. When the bulk of torus material reaches the innermost region close to a central black hole, a magnetically driven jet emerges. This magnetic jet is derived by vertically inflating toroidal fields (magnetic tower) and has a two-component structure: low-β (1) plasmas threaded with poloidal (vertical) fields are surrounded by those with toroidal fields. The collimation width of the jet depends on external pressure, the pressure of ambient medium; the weaker the external pressure is, the wider and the longer lasting becomes the jet. Unless the external pressure is negligible, the bipolar jet phase ceases after several dynamical timescales at the original torus position and a subsequent quasi-steady state starts. The black hole is surrounded by a quasi-spherical zone with highly inhomogeneous structure in which toroidal fields are dominant except near the rotation axis. Mass accretion takes place mainly along the equatorial plane. Comparisons with other MHD simulation results and observational implications are discussed.

Spectra from three-dimensional magneto-hydrodynamical accretion flows

Global three-dimensional (3D) magneto-hydrodynamical (MHD) simulations of accretion flow have been extensively performed recently. We calculate the emergent spectra from 3D MHD flows by the Monte Carlo radiative transfer simulations, and examine to what extent simulated MHD flows can account for the observed spectrum of Sagittarius A* (Sgr A*). It is found that the emergent spectrum from the inner part of the MHD flow (r&lt;10rs) is consistent with the observations in the X-ray flaring state of Sgr A*, where rs is the Schwarzschild radius. However, the MHD flow can not reproduce the spectrum in the quiescent state.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Magnetohydrodynamical Accretion Flows: Formation of Magnetic Tower Jet and Subsequent Quasi-Steady State

We present three-dimensional (3-D) magnetohydrodynamical (MHD) simulations of both radiatively inefficient accretion flow (RIAF) into and jet from black holes (BHs). When the bulk of torus material reaches the innermost region close to a central BH, a magnetically driven jet emerges. This magnetic jet is derived by vertically inflating toroidal fields ('magnetic tower') and has a two-component structure: low-β (< 1) plasmas threaded with poloidal fields are surrounded by that with toroidal fields. The collimation width of the jet depends on external pressure, pressure of ambient medium; the weaker the external pressure is, the wider and the longer-lasting becomes the jet. Unless the external pressure is negligible, the bipolar jet phase ceases after several dynamical timescales at the original torus position and a subsequent quasi-steady state starts.

The Magnetohydrodynamics of Convection‐dominated Accretion Flows

Radiatively inefficient accretion flows onto black holes are unstable due to both an outwardly decreasing entropy (`convection') and an outwardly decreasing rotation rate (the `magnetorotational instability'; MRI). Using a linear magnetohydrodynamic stability analysis, we show that long-wavelength modes are primarily destabilized by the entropy gradient and that such `convective' modes transport angular momentum inwards. Moreover, the stability criteria for the convective modes are the standard Hoiland criteria of hydrodynamics. By contrast, shorter wavelength modes are primarily destabilized by magnetic tension and differential rotation. These `MRI' modes transport angular momentum outwards. The convection-dominated accretion flow (CDAF) model, which has been proposed for radiatively inefficient accretion onto a black hole, posits that inward angular momentum transport and outward energy transport by long-wavelength convective fluctuations are crucial for determining the structure of the accretion flow. Our analysis suggests that the CDAF model is applicable to a magnetohydrodynamic accretion flow provided the magnetic field saturates at a sufficiently sub-equipartition value (plasma beta >> 1), so that long-wavelength convective fluctuations can fit inside the accretion disk. Numerical magnetohydrodynamic simulations are required to determine whether such a sub-equipartition field is in fact obtained.

Effects of magnetohydrodynamic accretion flows on global structure of a Kerr black-hole magnetosphere

In relation to black-hole models for central engines in active galactic nuclei, we discuss some effects of accreting matter on the global structure of the Kerr black-hole magnetosphere. Based on the steady axisymmetric magnetohydrodynamic (MHD) equations in Kerr geometry, we analytically study the general-relativistic Grad-Shafranov equation and the poloidal wind equation. The field line geometry is clearly given in the region where the plasma can be magnetically supported. Near the black hole, the magnetic support breaks down and the plasma begins to flow toward the event horizon. We consider the magnetohydrodynamic conditions for the plasma inflows which should pass through the Alfv\'en critical point. Then it is found that the accumulation of magnetic field lines threading the horizon is suppressed for the rapidly rotating holes. This result means that the global shape of the field lines and the MHD energy extraction from the rotating black hole can drastically change as the spin down of the black hole proceeds. We present an evolutionary model for the activity and the structure of the black-hole magnetosphere.