Mami Machida

Global Simulations of Differentially Rotating Magnetized Disks: Formation of Low-β Filaments and Structured Coronae

We present the results of three-dimensional global magnetohydrodynamic simulations of the Parker-shearing instability in a differentially rotating torus initially threaded by toroidal magnetic fields. An equilibrium model of a magnetized torus is adopted as an initial condition. When beta0=Pgas&solm0;Pmag approximately 1 at the initial state, magnetic flux buoyantly escapes from the disk and creates looplike structures similar to those in the solar corona. Inside the torus, the growth of nonaxisymmetric magnetorotational (or Balbus & Hawley) instability generates magnetic turbulence. Magnetic field lines are tangled on a small scale, but on a large scale they show low azimuthal wavenumber spiral structure. After several rotation periods, the system oscillates around a state with beta approximately 5. We found that magnetic pressure-dominated (beta<1) filaments are created in the torus. The volume filling factor of the region in which beta</=0.3 is 2%-10%. Magnetic energy release in such low-beta regions may lead to violent flaring activities in accretion disks and in galactic gas disks.

Global three-dimensional magnetohydrodynamic simulations of black hole accretion disks: X-ray flares in the plunging region

We present the results of three-dimensional global resistive magnetohydrodynamic (MHD) simulations of black hole accretion flows. General relativistic effects are simulated by using the pseudo-Newtonian potential. The initial state is an equilibrium model of a torus threaded by weak toroidal magnetic fields. As the magnetorotational instability (MRI) grows in the torus, mass accretes to the black hole by losing angular momentum. We found that in the innermost plunging region, nonaxisymmetric accretion flow creates bisymmetric spiral magnetic fields and current sheets. Mass accretion along the spiral channel creates one-armed spiral density distribution. Since the accreting matter carries in magnetic fields that are subsequently stretched and amplified as a result of differential rotation, current density increases inside the channel. Magnetic reconnection taking place in the current sheet produces slow-mode shock waves that propagate away from the reconnection site. Magnetic energy release in the innermost plunging region could be the origin of X-ray shots observed in black hole candidates. Numerical simulations reproduced soft X-ray excess preceding the peak of the shots, X-ray hardening at the peak of the shot, and hard X-ray time lags.

Formation of Magnetically Supported Disks during Hard-to-Soft Transitions in Black Hole Accretion Flows

We carried out three-dimensional global resistive magnetohydrodynamic simulations of the cooling instability in optically thin hot black hole accretion flows by assuming bremsstrahlung cooling. General relativistic effects are simulated using the pseudo-Newtonian potential. The cooling instability grows when the density of the accretion disk becomes sufficiently large. We found that as the instability grows the accretion flow changes from an optically thin, hot, gas pressure-supportedstate (low/hard state) to acooler, magnetically supported, quasi-steady state. During this transition, the magnetic pressure exceeds the gas pressure because the disk shrinks in the vertical direction while almost conserving the toroidal magnetic flux. Since further vertical contraction of the disk is suppressed by magnetic pressure, the cool disk stays in an optically thin, spectrally hard state. The magnetically supported disk exists for a time scale much longer than the thermal time scale, and comparable to the accretion time scale. We examined the stability of the magnetically supported disk analytically, assuming that the toroidal magnetic flux is conserved, and found it to be thermally and secularly stable. Our findings may explain why black hole candidates stay in luminous, hard state even when their luminosity exceeds the threshold for the onset of the cooling instability.

THERMAL EQUILIBRIA OF OPTICALLY THIN, MAGNETICALLY SUPPORTED, TWO-TEMPERATURE, BLACK HOLE ACCRETION DISKS

We obtained thermal equilibrium solutions for optically thin, two-temperature black hole accretion disks incorporating magnetic fields. The main objective of this study is to explain the bright/hard state observed during the bright/slow transition of galactic black hole candidates. We assume that the energy transfer from ions to electrons occurs via Coulomb collisions. Bremsstrahlung, synchrotron, and inverse Compton scattering are considered as the radiative cooling processes. In order to complete the set of basic equations, we specify the magnetic flux advection rate instead of ? = p gas/p mag. We find magnetically supported (low-?), thermally stable solutions. In these solutions, the total amount of the heating via the dissipation of turbulent magnetic fields goes into electrons and balances the radiative cooling. The low-? solutions extend to high mass accretion rates () and the electron temperature is moderately cool (T e ~ 108-109.5 K). High luminosities (0.1L Edd) and moderately high energy cutoffs in the X-ray spectrum (~50-200 keV) observed in the bright/hard state can be explained by the low-? solutions.

GLOBAL THREE-DIMENSIONAL MAGENTOHYDRODYNAMIC SIMULATIONS OF GALACTIC GASEOUS DISKS. I. AMPLIFICATION OF MEAN MAGNETIC FIELDS IN AN AXISYMMETRIC GRAVITATIONAL POTENTIAL

We carried out global three-dimensional resistive magnetohydrodynamic simulations of galactic gaseous disks to investigate how the galactic magnetic fields are amplified and maintained. We adopt a steady axisymmetric gravitational potential given by Miyamoto & Nagai and Miyamoto et al. As the initial condition, we assume a warm (T ~ 105 K) rotating gas torus centered at = 10 kpc threaded by weak azimuthal magnetic fields. Numerical results indicate that in differentially rotating galactic gaseous disks, magnetic fields are amplified due to magnetorotational instability and magnetic turbulence develops. After the amplification of magnetic energy saturates, the disk stays in a quasi-steady state. The mean azimuthal magnetic field increases with time and shows reversals with a period of 1 Gyr (2 Gyr for a full cycle). The amplitude of B near the equatorial plane is B ~ 1.5 μG at = 5 kpc. The magnetic fields show large fluctuations whose standard deviation is comparable to the mean field. The mean azimuthal magnetic field in the disk corona has a direction opposite to the mean magnetic field inside the disk. The mass accretion rate driven by the Maxwell stress is ~10-3 M☉ yr-1 at = 2.5 kpc when the mass of the initial torus is ~5 × 108 M☉. When we adopt an absorbing boundary condition at r = 0.8 kpc, the rotation curve obtained by numerical simulations almost coincides with the rotation curve of the stars and the dark matter. Thus, even when magnetic fields are not negligible for gas dynamics of a spiral galaxy, the galactic gravitational potential can be derived from observations of the rotation curve using the gas component of the disk.

THERMAL EQUILIBRIA OF MAGNETICALLY SUPPORTED BLACK HOLE ACCRETION DISKS

We present new thermal equilibrium solutions for optically thin and optically thick disks incorporating magnetic fields. The purpose of this paper is to explain the bright hard state and the bright/slow transition observed in the rising phases of outbursts in black hole candidates. On the basis of the results of three-dimensional magnetohydrodynamic simulations, we assume that magnetic fields inside the disk are turbulent and dominated by the azimuthal component and that the azimuthally averaged Maxwell stress is proportional to the total (gas, radiation, and magnetic) pressure. We prescribe the magnetic flux advection rate to determine the azimuthal magnetic flux at a given radius. Local thermal equilibrium solutions are obtained by equating the heating, radiative cooling, and heat advection terms. We find magnetically supported (β = (p gas + p rad)/p mag < 1), thermally stable solutions for both optically thin disks and optically thick disks, in which the heating enhanced by the strong magnetic field balances the radiative cooling. The temperature in a low-β disk (T ~ 107-1011K) is lower than that in an advection-dominated accretion flow (or radiatively inefficient accretion flow) but higher than that in a standard disk. We also study the radial dependence of the thermal equilibrium solutions. The optically thin, low-β branch extends to , where is the mass accretion rate and is the Eddington mass accretion rate, in which the temperature anticorrelates with the mass accretion rate. Thus, optically thin low-β disks can explain the bright hard state. Optically thick, low-β disks have the radial dependence of the effective temperature T eff –3/4. Such disks will be observed as staying in a high/soft state. Furthermore, limit cycle oscillations between an optically thick low-β disk and a slim disk will occur because the optically thick low-β branch intersects with the radiation pressure dominated standard disk branch. These limit cycle oscillations will show a smaller luminosity variation than that between a standard disk and a slim disk.

DYNAMO ACTIVITIES DRIVEN BY MAGNETOROTATIONAL INSTABILITY AND THE PARKER INSTABILITY IN GALACTIC GASEOUS DISKS

We carried out global three-dimensional magnetohydrodynamic simulations of dynamo activities in galactic gaseous disks without assuming equatorial symmetry. Numerical results indicate the growth of azimuthal magnetic fields non-symmetric to the equatorial plane. As the magnetorotational instability (MRI) grows, the mean strength of magnetic fields is amplified until the magnetic pressure becomes as large as 10% of the gas pressure. When the local plasma β (=p gas/p mag) becomes less than 5 near the disk surface, magnetic flux escapes from the disk by the Parker instability within one rotation period of the disk. The buoyant escape of coherent magnetic fields drives dynamo activities by generating disk magnetic fields with opposite polarity to satisfy the magnetic flux conservation. The flotation of the azimuthal magnetic flux from the disk and the subsequent amplification of disk magnetic field by the MRI drive quasi-periodic reversal of azimuthal magnetic fields on a timescale of 10 rotation periods. Since the rotation speed decreases with radius, the interval between the reversal of azimuthal magnetic fields increases with radius. The rotation measure computed from the numerical results shows symmetry corresponding to a dipole field.

Formation of galactic center magnetic loops

A survey of the molecular clouds in the Galaxy with the NANTEN mm telescope has discovered molecular loops in the galactic center region. They show monotonic gradients of the line-of-sight velocity along the loops and large velocity dispersions towards their foot-points. It is suggested that these loops can be explained in terms of a buoyant rise of magnetic loops due to a Parker instability. We carried out global three-dimensional magnetohydrodynamic simulations of the gas disk in the galactic center. The gravitational potential was approximated by an axisymmetric potential proposed by Miyamoto and Nagai (1975, PASJ, 27, 533). At the initial state, we assumed aw arm (� 10 4 K) gas torus threaded by azimuthal magnetic fields. Self-gravity and radiative cooling of the gas were ignored. We found that buoyantly rising magnetic loops are formed above the differentially rotating, magnetically turbulent disk. By analyzing the results of global MHD simulations, we identified individual loops, about 180 in the upper half of the disk, and studied their statistical properties, such as their length, width, height, and velocity distributions along the loops. The typical length and height of a loop are 1 kpc and 200 pc, respectively. The line-ofsight velocity changes linearly along a loop, and shows large dispersions around the foot-points. Numerical results indicate that loops emerge preferentially from the region where the magnetic pressure is large. We argue that these properties are consistent with those of molecular loops discovered by NANTEN.

The Polish doughnuts revisited I. The angular momentum distribution and equipressure surfaces

We construct a new family of analytic models of black hole accretion disks in dynamical equilibria. Our construction is based on assuming distributions of angular momentum and entropy. For a particular choice of the distribution of angular momentum, we calculate the shapes of equipressure surfaces. The equipressure surfaces we find are similar to those in thick, slim and thin disks, and to those in ADAFs.

Steady models of optically thin, magnetically supported black hole accretion disks

We obtained steady solutions of optically thin, single-temperature, magnetized black hole accretion disks, assuming thermal bremsstrahlung cooling. Based on the results of 3D MHD simulations of accretion disks, we assumed that the magnetic fields inside the disk are turbulent and dominated by an azimuthal component. We decomposed the magnetic fields into an azimuthally averaged mean field and fluctuating fields. We also assumed that the azimuthally averaged Maxwel ls tress i sp roportional to the total pressure. The radial advection rate of the azimuthal magnetic flux, P Φ ,i s prescribed as being proportional to