arXiv subjects

R. Matsumoto

Publications and source records attributed to R. Matsumoto.

30 records · Page 2Linked to original sources

arXiv2009

Thermal Equilibria of Magnetically Supported, Black Hole Accretion Disks

We present new thermal equilibrium solutions for optically thin and 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 BHCs. On the basis of the results of 3D MHD 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 pressure. We prescribe the magnetic flux advection rate to determine the azimuthal magnetic flux at a given radius. We find magnetically supported, thermally stable solutions for both optically thin and thick disks, in which the heating enhanced by the strong magnetic field balances the radiative cooling. The temperature in a low-$β$ disk is lower than that in an ADAF/RIAF 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 $ \dot M \gtrsim 0.1 {\dot M}_{\rm Edd}$, in which the temperature anti-correlates with the mass accretion rate. Thus optically thin low-$β$ disks can explain the bright hard state. Optically thick, low-$β$ disks have the radial dependence of the effective temperature $T_{\rm eff} \propto \varpi^{-3/4}$. 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.

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arXiv2007

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 azimuthal component. We decomposed magnetic fields into an azimuthally averaged mean field and fluctuating fields. We also assumed that the azimuthally averaged Maxwell stress is proportional to the total pressure. The radial advection rate of the azimuthal magnetic flux $\dot Φ$ is prescribed as being proportional to $\varpi^{- ζ}$, where $\varpi$ is the radial coordinate and $ζ$ is a parameter which parameterizes the radial variation of $\dot Φ$. We found that when accretion rate $\dot M$ exceeds the threshold for the onset of the thermal instability, a magnetic pressure dominated new branch appears. Thus the thermal equilibrium curve of optically thin disk has a 'Z'-shape in the plane of surface density and temperature. This indicates that as the mass accretion rate increases, a gas pressure dominated optically thin hot accretion disk undergoes a transition to a magnetic pressure dominated, optically thin cool disk. This disk corresponds to the X-ray hard, luminous disk in black hole candidates observed during the transition from a low/hard state to a high/soft state. We also obtained global steady transonic solutions containing such a transition layer.

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arXiv2004

The Acceleration Mechanism of Resistive MHD Jets Launched from Accretion Disks

We analyzed the results of non-linear resistive magnetohydrodynamical (MHD) simulations of jet formation to study the acceleration mechanism of axisymmetric, resistive MHD jets. The initial state is a constant angular momentum, polytropic torus threaded by weak uniform vertical magnetic fields. The time evolution of the torus is simulated by applying the CIP-MOCCT scheme extended for resistive MHD equations. We carried out simulations up to 50 rotation period at the innermost radius of the disk created by accretion from the torus. The acceleration forces and the characteristics of resistive jets were studied by computing forces acting on Lagrangian test particles. Since the angle between the rotation axis of the disk and magnetic field lines is smaller in resistive models than in ideal MHD models, magnetocentrifugal acceleration is smaller. The effective potential along a magnetic field line has maximum around $z \sim 0.5r_0$ in resistive models, where $r_0$ is the radius where the density of the initial torus is maximum. Jets are launched after the disk material is lifted to this height by pressure gradient force. Even in this case, the main acceleration force around the slow magnetosonic point is the magnetocentrifugal force. The power of the resistive MHD jet is comparable to the mechanical energy liberated in the disk by mass accretion. Joule heating is not essential for the formation of jets.

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arXiv2002

Gravitational-Wave Radiation from Magnetized Accretion Disks

The detectability of gravitational wave (GW) radiation from accretion disks is discussed based on various astrophysical contexts. In order to emit GW radiation, the disk shape should lose axial symmetry. We point out that a significant deformation is plausible in non-radiative hot accretion disks because of enhanced magnetic activity, whereas it is unlikely for standard-type cool disks. We have analyzed the 3D magnetohydrodynamical (MHD) simulation data of magnetized accretion flow, finding non-axisymmetric density patterns. The corresponding ellipticity is $ε\sim 0.01$. The expected time variations of GW radiation are overall chaotic, but there is a hint of quasi-periodicity. GW radiation has no interesting consequence, however, in the case of close binaries, because of very tiny disk masses. GW radiation is not significant, either, for AGN because of very slow rotation velocities. The most promising case can be found in gamma-ray bursts or supernovae, in which a massive torus (or disk) with a solar mass or so may be formed around a stellar-mass compact object as the result of a merger of compact objects, or by the fallback of exploded material towards the center in a supernova. Although much more intense GW radiation is expected before the formation of the torus, the detection of GW radiation in the subsequent accretion phase is of great importance, since it will provide a good probe to investigating their central engines.

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arXiv2001

Magnetohydrodynamic Simulations of Accretion Disks around a Weakly Magnetized Neutron Star in Strong Gravity

We carried out two dimensional high-resolution magnetohydrodynamic (MHD) simulations of an accretion disk around a weakly magnetized neutron star. General relativistic effects are taken into account by using pseudo-Newtonian potential of Paczýnski, B., P. J. Witta (1980). When magnetic loops connect the neutron star and the accretion disk, the twist injection from the disk or from the rotating neutron star triggers expansion of the loops. Since the expanding magnetic loops prevent inflow toward the magnetic poles of the neutron star, disk matter accumulates on the boundary between the magnetosphere and the disk. Magnetic reconnection taking place in the loops creates a channel along which the disk matter can accrete and unloads the magnetosphere. This process produces quasi-periodic variation of the accretion flow in the innermost region of the disk. We found two kinds of oscillations. One is the magnetospheric oscillation regulated by magnetic reconnection. The other is the radial disk oscillation. The typical frequency of the oscillations is 100 Hz to 2 kHz. Furthermore, we predict that QPO sources inevitably accompany X-ray flares by magnetic reconnection and bipolar outflows of hot X-ray emitting plasma similar to the optical jets in protostars.

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arXiv2000

Resistive Magnetohydrodynamics of Jet Formation and Magnetically Driven Accretion

We carried out 2.5-dimensional resistive magnetohydrodynamic simulations to study the effects of magnetic diffusivity on magnetically driven mass accretion and jet formation. We found that (1) when the normalized magnetic diffusivity, is small, mass accretion and jet formation take place intermittently; (2) when diffusivity is middle, the system evolves toward a quasi-steady state; and the system evolves toward a quasi-steady state; and (3) when diffusivity is large, the accretion/mass outflow rate decreases with diffusivity and approaches 0. The results of these simulations indicate magnetic braking provide a mass accretion rate which is sufficient to explain the activity of AGNs.

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arXiv2000

Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk

The stability of nonaxisymmetric perturbations in differentially rotating astrophysical accretion disks is analyzed by fully incorporating the properties of shear flows. We verify the presence of discrete unstable eigenmodes with complex and pure imaginary eigenvalues, without any artificial disk edge boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the mathematical theory of a non-self-adjoint system, we investigate the nonlocal behavior of eigenmodes in the vicinity of Alfven singularities at omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of discrete eigenmodes is discussed and the magnetic field and wavenumber dependence of the growth rate are obtained. Exponentially growing modes are present even in a region where the local dispersion relation theory claims to have stable eigenvalues. The velocity field created by an eigenmode is obtained, which explains the anomalous angular momentum transport in the nonlinear stage of this stability.

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arXiv1999

Temporal 1/f^αFluctuations from Fractal Magnetic Fields in Black Hole Accretion Flow

Rapid fluctuation with a frequency dependence of $1/f^α$ (with $α\simeq 1 - 2$) is characteristic of radiation from black-hole objects. Its origin remains poorly understood. We examine the three-dimensional magnetohydrodynamical (MHD) simulation data, finding that a magnetized accretion disk exhibits both $1/f^α$ fluctuation (with $α\simeq 2$) and a fractal magnetic structure (with the fractal dimension of $D \sim 1.9$). The fractal field configuration leads reconnection events with a variety of released energy and of duration, thereby producing $1/f^α$ fluctuations.

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arXiv1999

Global Simulations of Differentially Rotating Magnetized Disks : Formation of Low-Beta Filaments and Structured Corona

We present the results of three-dimensional global magnetohydrodynamic (MHD) simulations of the Parker-shearing instability in a differentially rotating torus initially threaded by toroidal magnetic fields. An equilibrium model of magnetized torus is adopted as an initial condition. When $β_0 = P_{\rm gas}/P_{\rm mag} \sim 1$ at the initial state, magnetic flux buoyantly escapes from the disk and creates loop-like structures similar to those in the solar corona. Inside the torus, growth of non-axisymmetric magneto-rotational (or Balbus & Hawley) instability generates magnetic turbulence. Magnetic field lines are tangled in small scale but in large scale they show low azimuthal wave number spiral structure. After several rotation period, the system oscillates around a state with $β\sim 5$. We found that magnetic pressure dominated ($β< 1$) filaments are created in the torus. The volume filling factor of the region where $β\leq 0.3$ is 2-10%. Magnetic energy release in such low-$β$ regions may lead to violent flaring activities in accretion disks and in galactic gas disks.

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arXiv1996

X-Ray Flares and Mass Outflows Driven by Magnetic Interaction between a Protostar and its Surrounding Disk

We propose a model of hard X-ray flares in protostars observed by ASCA satellite. Assuming that the dipole magnetic field of the protostar threads the protostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD) simulations of the disk-star interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. As the twist accumulates, magnetic loops expand and finally approach to the open field configuration. A current sheet is formed inside the expanding loops. In the presence of resistivity, magnetic reconnection takes place in the current sheet. Outgoing magnetic island and post flare loops are formed as a result of the reconnection. The time scale of this `flare' is the order of the rotation period of the disk. The released magnetic energy partly goes into the thermal energy and heats up the flaring plasma up to $10^8$ K. The length of the flaring loop is several times of the radius of the central star, consistent with observations. The speed of the hot plasmoid ejected by the reconnection is $200-400 $ km s$^{-1}$ when the footpoint of the loop is at 0.03 AU from 1 M$_\odot$ protostar. The hot plasma outflow can explain the speed and mass flow rate of optical jets. Dense, cold, magnetically accelerated wind ($v \sim 150-250$ km s$^{-1}$) emanates from the surface of the disk along the partially open magnetic field lines threading the disk. This dense, cold wind may correspond to high velocity neutral winds.

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arXiv1996

X-Ray Fluctuations from Locally Unstable Advection-Dominated Disks

The response of advection-dominated accretion disks to local disturbances is examined by one-dimensional numerical simulations. It is generally believed that advection-dominated disks are thermally stable. We, however, find that any disurbance added onto accretion flow at large radii does not decay so rapidly that it can move inward with roughly the free-fall velocity. Although disturbances continue to be present, the global disk structure will not be modified largely. This can account for persistent hard X-ray emission with substantial variations observed in active galactic nuclei and stellar black hole candidates during the hard state. Moreover, when the disturbance reaches the innermost parts, an acoustic wave emerges, propagating outward as a shock wave. The resultant light variation is roughly (time) symmetric and is quite reminiscent of the observed X-ray shots of Cygnus X-1.

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arXiv1992

Giant Molecular Cloud Formation through the Parker Instability in a Skewed Magnetic Field

The effect of the magnetic skew on the Parker instability is investigated by means of the linear stability analysis for a gravitationally stratified gas layer permeated by a horizontal magnetic field. When the magnetic field is skewed (i.e., the field line direction is a function of the height), the wavelength of the most unstable mode is $ λ\; \sim \; 10 H $ where $ H $ is the pressure scale height. The growth rate of the short wavelength modes is greatly reduced when the gradient in the magnetic field direction exceeds 0.5 radian per scale height. Our results indicate that the Parker instability in a skewed magnetic field preferentially forms large scale structures like giant molecular clouds.

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