松元 亮治

Abstract Strong soft X-ray emission called soft X-ray excess is often observed in luminous active galactic nuclei (AGN). It has been suggested that the soft X-rays are emitted from a warm (T = 10⁶ ∼ 10⁷ K) region that is optically thick for the Thomson scattering (warm Comptonization region). Motivated by the recent observations that soft X-ray excess appears in changing look AGN (CLAGN) during the state transition from a dim state without broad emission lines to a bright state with broad emission lines, we performed global three-dimensional radiation magnetohydrodynamic simulations, assuming that the mass accretion rate increases and becomes around 10% of the Eddington accretion rate. The simulation successfully reproduces a warm, Thomson-thick region outside the hot radiatively inefficient accretion flow near the black hole. The warm region is formed by efficient radiative cooling due to inverse Compton scattering. The calculated luminosity 0.01−0.08 L _Edd is consistent with the luminosity of CLAGN. We also found that the warm Comptonization region is well described by the steady model of magnetized disks supported by azimuthal magnetic fields. When the antiparallel azimuthal magnetic fields supporting the radiatively cooled region reconnect around the equatorial plane of the disk, the temperature of the region becomes higher by releasing the magnetic energy transported to the region.

Based mainly on X-ray observations, we study the interactions between the intracluster medium (ICM) in clusters of galaxies and their member galaxies. Through (magneto)hydrodynamic and gravitational channels, moving galaxies are expected to drag the ICM around them, and then transfer some fraction of their dynamical energies on cosmological timescales to the ICM. This hypothesis is in line with several observations, including the possible cosmological infall of galaxies toward the cluster center, found over redshifts of <italic>z</italic> ∼ 1 to <italic>z</italic> ∼ 0. Further assuming that the energy lost by these galaxies is first converted into ICM turbulence and then dissipated, this picture can explain the subsonic and uniform ICM turbulence, measured with <italic>Hitomi</italic> in the core region of the Perseus cluster. The scenario may also explain several other unanswered problems regarding clusters of galaxies, such as what prevents the ICM from underoing the expected radiative cooling, how the various mass components in nearby clusters have attained different radial distributions, and how a thermal stability is realized between hot and cool ICM components that co-exist around cD galaxies. This view is also considered to pertain to the general scenario of galaxy evolution, including their environmental effects.

<title>ABSTRACT</title> We present the results of two-temperature magnetohydrodynamic simulations of the propagation of sub-relativistic jets of active galactic nuclei. The dependence of the electron and ion temperature distributions on the fraction of electron heating, fe, at the shock front is studied for fe = 0, 0.05, and 0.2. Numerical results indicate that in sub-relativistic, rarefied jets, the jet plasma crossing the terminal shock forms a hot, two-temperature plasma in which the ion temperature is higher than the electron temperature. The two-temperature plasma expands and forms a backflow referred to as a cocoon, in which the ion temperature remains higher than the electron temperature for longer than 100 Myr. Electrons in the cocoon are continuously heated by ions through Coulomb collisions, and the electron temperature thus remains at Te &amp;gt; 109 K in the cocoon. X-ray emissions from the cocoon are weak because the electron number density is low. Meanwhile, X-rays are emitted from the shocked intracluster medium (ICM) surrounding the cocoon. Mixing of the jet plasma and the shocked ICM through the Kelvin–Helmholtz instability at the interface enhances X-ray emissions around the contact discontinuity between the cocoon and shocked ICM.

A possibility of time-delayed radio brightenings of Sgr A* triggered by the pericenter passage of the G2 cloud is studied by carrying out global three-dimensional magnetohydrodynamic simulations, taking into account the radiative cooling of the tidal debris of the G2 cloud. Magnetic fields in the accretion flow are strongly perturbed and reorganized after the passage of G2. We have found that the magnetic energy in the accretion flow increased by a factor of 3-4 in 5-10 yr after the passage of G2 through a dynamo mechanism driven by the magneto-rotational instability. Since this B-field amplification enhances the synchrotron emission from the disk and the outflow, the radio and the infrared luminosity of Sgr A* are expected to increase some time, around 2020. The time delay of the radio brightening enables us to determine the rotation axis of the preexisting disk.

The formation mechanism of CO clouds observed with the NANTEN2 and Mopra telescopes toward the stellar cluster Westerlund 2 is studied by 3D magnetohydrodynamic simulations, taking into account the interstellar cooling. These molecular clouds show a peculiar shape composed of an arc-shaped cloud on one side of the TeV gamma-ray source HESS J1023-575 and a linear distribution of clouds ( jet clouds) on the other side. We propose that these clouds are formed by the interaction of a jet with clumps of interstellar neutral hydrogen (H I). By studying the dependence of the shape of dense cold clouds formed by shock compression and cooling on the filling factor of H I clumps, we found that the density distribution of H I clumps determines the shape of molecular clouds formed by the jet-cloud interaction: arc clouds are formed when the filling factor is large. On the other hand, when the filling factor is small, molecular clouds align with the jet. The jet propagates faster in models with small filling factors.

We carried out 2.5-dimensional resistive MHD simulations to study the formation mechanism of molecular loops observed by Fukui et al. in the Galactic central region. Since it is hard to form molecular loops by lifting up dense molecular gas, we study the formation mechanism of molecular gas in rising magnetic arcades. This model is based on the in situ formation model of solar prominences, in which prominences are formed by cooling instability in helical magnetic flux ropes formed by imposing converging and shearing motion at footpoints of the magnetic arch anchored to the solar surface. We extended this model to Galactic center scale (a few hundreds of parsecs). Numerical results indicate that magnetic reconnection taking place in the current sheet that formed inside the rising magnetic arcade creates dense blobs confined by the rising helical magnetic flux ropes. Thermal instability taking place in the flux ropes forms dense molecular filaments floating at high Galactic latitude. The mass of the filament increases with time and can exceed 10(5) M-circle dot.

Various observations show peculiar features in the Galactic Center region, such as loops and filamentary structure. It is still unclear how such characteristic features are formed. Magnetic field is believed to play very important roles in the dynamics of gas in the Galaxy Center. Suzuki et al. (2015) performed a global magneto-hydrodynamical simulation focusing on the Galactic Center with an axisymmetric gravitational potential and claimed that non-radial motion is excited by magnetic activity. We further analyzed their simulation data and found that vertical motion is also excited by magnetic activity. In particular, fast down flows with speed of ∼100 km/s are triggered near the footpoint of magnetic loops that are buoyantly risen by Parker instability. These downward flows are accelerated by the vertical component of the gravity, falling along inclined field lines. As a result, the azimuthal and radial components of the velocity are also excited, which are observed as high velocity features in a simulated position-velocity diagram. Depending on the viewing angle, these fast flows will show a huge variety of characteristic features in the position-velocity diagram.

By performing a global magnetohydrodynamical (MHD) simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic centre (GC) region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. In addition, Parker instability (magnetic buoyancy) creates vertical magnetic structure, which would correspond to observed molecular loops, and frequently excited vertical flows. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which would contribute to the outflow from the bulge.

By performing a global magnetohydrodynamical simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic centre region. The radial distribution of the rotation frequency in the bulge region is not mono-tonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches greater than or similar to 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. In addition, the magnetic pressure-gradient force also drives radial flows in a similar manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. This is a natural extension into the central few 100 pc of the magnetic activity, which is observed as molecular loops at radii from a few 100 pc to 1 kpc. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which we discuss from a viewpoint of the outflow from the bulge.

The formation mechanism of the jet-aligned CO clouds found by NANTEN CO observations is studied by magnetohydrodynamical (MHD) simulations taking into account the cooling of the interstellar medium. Motivated by the association of the CO clouds with the enhancement of Hi gas density, we carried out MHD simulations of the propagation of a supersonic jet injected into the dense Hi gas. We found that the Hi gas compressed by the bow shock ahead of the jet is cooled down by growth of the cooling instability triggered by the density enhancement. As a result, a cold dense sheath is formed around the interface between the jet and the Hi gas. The radial speed of the cold, dense gas in the sheath is a few km s(-1) almost independent of the jet speed. Molecular clouds can be formed in this region. Since the dense sheath wrapping the jet reflects waves generated in the cocoon, the jet is strongly perturbed by the vortices of the warm gas in the cocoon, which breaks up the jet and forms a secondary shock in the Hi-cavity drilled by the jet. The particle acceleration at the shock can be the origin of radio and X-ray filaments observed near the eastern edge of the W50 nebula surrounding the galactic jet source SS433.

We have made new CO observations of two molecular clouds, which we call "jet" and "arc" clouds, toward the stellar cluster Westerlund 2 and the TeV gamma-ray source HESS J1023-575. The jet cloud shows a linear structure from the position of Westerlund 2 on the east. In addition, we have found a new counter jet cloud on the west. The arc cloud shows a crescent shape in the west of HESS J1023-575. A sign of star formation is found at the edge of the jet cloud and gives a constraint on the age of the jet cloud to be similar to Myr. An analysis with the multi CO transitions gives temperature as high as 20 K in a few places of the jet cloud, suggesting that some additional heating may be operating locally. The new TeV gamma-ray images by H.E.S.S. correspond to the jet and arc clouds spatially better than the giant molecular clouds associated with Westerlund 2. We suggest that the jet and arc clouds are not physically linked with Westerlund 2 but are located at a greater distance around 7.5 kpc. A microquasar with long-term activity may be able to offer a possible engine to form the jet and arc clouds and to produce the TeV gamma-rays, although none of the known microquasars have a Myr age or steady TeV gamma-rays. Alternatively, an anisotropic supernova explosion which occurred similar to Myr ago may be able to form the jet and arc clouds, whereas the TeV gamma-ray emission requires a microquasar formed after the explosion.

We carried out large-scale (4 × 2 degree) CO multi-line observations toward the central molecular zone (CMZ) in the Galactic center (GC) with the NANTEN2 4m telescope and mapped several diffuse molecular features located at relatively high Galactic latitudes above 0°.6. These high-latitude features are composed of diffuse molecular halo gas and molecular filaments according to their morphological aspects. Their high velocities and high intensity ratios between 12CO J = (2-1) and J = (1-0) clearly indicate their location in the GC, and their total mass amount to ~10% of that of the CMZ. We discuss that magnetic field is a possible mechanism of these high-latitude molecular features lifting up toward high galactic latitude. Copyright © International Astronomical Union 2014.

We have discovered two molecular features at radial velocities of -35 km s(-1) and 0 km s(-1) toward the infrared Double Helix Nebula (DHN) in the Galactic center with NANTEN2. The two features show good spatial correspondence with the DHN. We have also found two elongated molecular ridges at these two velocities distributed vertically to the Galactic plane over 0.degrees 8. The two ridges are linked by broad features in velocity and are likely connected physically with each other. The ratio between the (CO)-C-12 J=2-1 and J=1-0 transitions is 0.8 in the ridges which is larger than the average value 0.5 in the foreground gas, suggesting the two ridges are in the Galactic center. An examination of the K band extinction reveals a good coincidence with the CO 0 km s(-1) ridge and is consistent with a distance of 8 +/- 2 kpc. We discuss the possibility that the DHN was created by a magnetic phenomenon incorporating torsional Alfven waves launched from the circum-nuclear disk and present a first estimate of the mass and energy involved in the DHN.

A tidal disruption event by a supermassive black hole in Swift J1644+57 can trigger limit-cycle oscillations between a supercritically accreting X-ray bright state and a subcritically accreting X-ray dim state. The time evolution of debris gas around a black hole with mass M = 10(6) M-circle dot is studied by performing axisymmetric, two-dimensional radiation hydrodynamic simulations. We assume the alpha-prescription of viscosity, in which the viscous stress is proportional to the total pressure. The mass supply rate from the outer boundary was assumed to be (M) over dot(supply) = 100 L-Edd/c(2), where L-Edd is the Eddington luminosity, and c is the light speed. Since the mass accretion rate decreases inward by outflows driven by radiation pressure, the state transition from a supercritically accreting slim disk state to a subcritically accreting Shakura-Sunyaev disk starts from the inner disk, and propagates outward on a timescale of one day. The sudden drop of the X-ray flux observed in Swift J1644+57 in 2012 August can be explained by this transition. As long as (M) over dot(supply) exceeds the threshold for the existence of a radiation pressure dominant disk, the accumulation of accreting gas in the subcritically accreting region triggers the transition from a gas pressure dominant Shakura-Sunyaev disk to a slim disk. This transition takes place at t similar to 50/(alpha/0.1) d after the X-ray darkening. We expect that if alpha &gt; 0.01, X-ray emission with luminosity &gt;= 10(44) erg s(-1) and jet ejection will revive in Swift J1644+57 in 2013-2014.

We carried out three-dimensional magnetohydrodynamic simulations to study the effects of plasma viscosity on the formation of sharp discontinuities of density and temperature distributions, cold fronts, in clusters of galaxies. By fixing the gravitational potential that confines the cool, dense plasma in a moving subcluster, we simulated its interaction with the hot, lower density plasma around the subcluster. At the initial state, the intracluster medium (ICM) is assumed to be threaded by uniform magnetic fields. The enhancement of plasma viscosity along the direction of magnetic fields is incorporated as anisotropic viscosity depending on the direction of magnetic fields. We found that the Kelvin-Helmholtz instability at the surface of the subcluster grows even in models with anisotropic viscosity, because its effects on the velocity shear across the magnetic field lines are suppressed. We also found that magnetic fields around the interface between the subcluster and ICM are amplified even in the presence of viscosity, while magnetic fields behind the subcluster are amplified up to beta(-1) similar to 0.01 in models with viscosity, whereas they are amplified up to beta(-1) similar to 0.1 in models without viscosity, where beta is the ratio of gas pressure to magnetic pressure.

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 beta (= 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.

Radiation spectra of supercritical black hole accretion flows are computed using a Monte Carlo method by post-processing the results of axisymmetric radiation hydrodynamic simulations. We take into account thermal/bulk Comptonization, free-free absorption, and photon trapping. We found that a shock-heated region (similar to 10(8) K) appears at the funnel wall near the black hole where the supersonic inflow is reflected by the centrifugal barrier of the potential. Both thermal and bulk Comptonization significantly harden photon spectra although most of the photons upscattered above 40 keV are swallowed by the black hole due to the photon trapping. When the accretion rate onto the black hole is (M) over dot approximate to 200L(E)/c(2), where L-E is the Eddington luminosity, the spectrum has a power-law component which extends up to similar to 10 keV by upscattering of photons in the shock-heated region. In higher mass accretion rates, the spectra roll over around 5 keV due to downscattering of the photons by cool electrons in the dense outflow surrounding the jet. Our results are consistent with the spectral features of ultraluminous X-ray sources, which typically show either a hard power-law component extending up to 10 keV or a rollover around 5 keV. We found that the spectrum of NGC 1313 X-2 is quite similar to the spectrum numerically obtained for high accretion rate ((M) over dot approximate to 1000L(E)/c(2)) source observed with low viewing angle (i = 10 degrees-20 degrees). Our numerical results also demonstrate that the face-on luminosity of supercritically accreting stellar mass black holes (10 M-circle dot) can significantly exceed 10(40) erg s(-1).

We present global solutions of optically thin, two-temperature black hole accretion disks incorporating magnetic fields. We assume that the (omega) over bar phi-component of the Maxwell stress is proportional to the total pressure, and prescribe the radial dependence of the magnetic flux advection rate in order to complete the set of basic equations. We obtained magnetically supported (low-beta) disk solutions, whose luminosity exceeds the maximum luminosity for an advection-dominated accretion flow (ADAF), L greater than or similar to 0.4 alpha L-2(Edd), where L-Edd is the Eddington luminosity. The accretion flow is composed of the outer ADAF, a luminous hot accretion flow (LHAF) inside the transition layer from the outer ADAF to the low-beta disk, the low-beta disk, and the inner ADAF. The low-beta disk region becomes wider as the mass-accretion rate increases further. In the low-beta disk, the magnetic heating balances the radiative cooling, and the electron temperature decreases from similar to 10(9.5) K to similar to 10(8) K as the luminosity increases. These results are consistent with the anti-correlation between the energy cutoff in X-ray spectra (hence the electron temperature) and the luminosity when L greater than or similar to 0.1 L-Edd, observed in the bright/hard state during the bright hard-to-soft transitions of transient outbursts in galactic black hole candidates.

We obtained self-similar solutions of relativistically expanding magnetic loops by assuming axisymmetry and a purely radial flow. The stellar rotation and the magnetic fields in the ambient plasma are neglected. We include the Newtonian gravity of the central star. These solutions are extended from those in our previous work by taking into account discontinuities such as the contact discontinuity and the shock. The global plasma flow consists of three regions, the outflowing region, the post-shocked region and the ambient plasma. They are divided by two discontinuities. The solutions are characterized by the radial velocity, which plays a role of the self-similar parameter in our solutions. The shock Lorentz factor gradually increases with radius. It can be approximately represented by the power of radius with the power-law index of 0.25. We also carried out magnetohydrodynamic (MHD) simulations of the evolution of magnetic loops to study the stability and the generality of our analytical solutions. We used the analytical solutions as the initial condition and the inner boundary conditions. We confirmed that our solutions are stable over the simulation time and that numerical results nicely recover the analytical solutions. We then carried out numerical simulations to study the generality of our solutions by changing the power-law index delta of the ambient plasma density (rho 0) proportional to r(-delta). We alter the power-law index delta from delta similar or equal to 3.5 in the analytical solutions. The analytical solutions are used as the initial conditions inside the shock in all simulations. We observed that the shock Lorentz factor increases with time when the power-law index is larger than 3, while it decreases with time when the power-law index is smaller than 3. The shock Lorentz factor Gamma(s) can be expressed as Gamma(s) proportional to((delta-3)/2) where delta is the power-law index of the ambient plasma. These results are consistent with the analytical studies by Shapiro.

We have carried out (CO)-C-12 (J = 2-1) and (CO)-C-12 (J = 3-2) observations at spatial resolutions of 1.0-3.8 pc toward the entirety of loops 1 and 2 and part of loop 3 in the Galactic center with NANTEN2 and ASTE. These new results have revealed detailed distributions of the molecular gas and the line intensity ratio of the two transitions, R3-2/2-1. In the three loops, R3-2/2-1, is in a range from 0.1 to 2.5 with a peak at similar to 0.7, while that in the disk molecular gas is in a range from 0.1 to 1.2 with a peak at 0.4. This supports that the loops are more highly excited than the disk molecular gas. An LVG analysis of three transitions, (CO)-C-12 J = 3-2 and 2-1 and (CO)-C-13 J = 2-1, toward six positions in loops 1 and 2 shows that the density and temperature are in the range 10(2.2)-10(4.7) cm(-3) and 15-100 K or higher, respectively. Three regions, extended by 50-100 pc in the loops, tend to have higher excitation conditions, as characterized by R3-2/2-1 greater than 1.2. The highest ratio of 2.5 is found in the most developed foot points between loops 1 and 2. This is interpreted that the foot points indicate strongly shocked conditions, as inferred from their large linewidths of 50-100 km s(-1), confirming a suggestion by Toni et al. (2010, PASJ, 62, 675). The other two regions outside the foot points suggest that the molecular gas is heated up by some additional heating mechanisms, possibly including magnetic reconnection. A detailed analysis of four foot points has shown a U shape, an L shape or a mirrored-L shape in the b-nu distribution. It is shown that a simple kinematical model that incorporates global rotation and expansion of the loops is able to explain these characteristic shapes.

Fukui et al. (2006) discovered two molecular loops in the Galactic center that are likely created by magnetic flotation due to the Parker instability. The magnetic flotation model offers another interpretation of the origin of the high temperature and large velocity dispersion of molecular gas in the Galactic center. Here, we present details of the two loops based on NANTEN CO(J=1-0) data sets.

We carried out multi-transition CO observations of the footpoint of the loops 1 and 2 discovered by Fukui et al. (2006). The aim of the observations is to clarify the detailed distributions of the gas and estimate the temperature and density of the shocked gas inside the footpoint. As a result, we find "U-shapes" in the footpoint and discuss that the U shapes indicate downflows of the gas from loops 1 and 2 that merge at the footpoint. We also find that a broad feature in the U shape shows the highest temperature of similar to&gt;100 K. Compared with the study on the solar surface, magnetic reconnection is a possible candidate to explain the hottest clump and U shape simultaneously.

Loop-like molecular structures have been observed in the Galactic center region with the NANTEN millimeter telescope (Fukui et al. 2006). They considered that loops are explained in terms of the buoyant rise of magnetic loops due to the Parker instability, because the estimated kinematic energy of loops exceed 1052 ergs and they could not find any infrared sources. We have carried out global three-dimensional magneto-hydrodynamic simulations of the gas disk in the Galactic center. The gravitational potential is approximated by the axisymmetric potential proposed by Miyamoto Sz Nagai (1975). In the initial state, we assume a warm (10(4)K) gas torus threaded by azimuthal magnetic fields. Radiative cooling of the gas is ignored. We ignored the self-gravity of the gas. We found that buoyantly rising magnetic loops are formed above the differentially rotating, magnetically turbulent disk. Typical length and height of a loop are 1 kpc and 200 pc, respectively.

Fukui et al. (2006, Science, 314, 106) discovered two huge molecular loops in the galactic center located at (l, b) similar or equal to (355 degrees-359 degrees, 0 degrees-2 degrees) in a large velocity range of -180-40 km s(-1). Following the discovery, we present detailed observational properties of the two loops based on NANTEN (CO)-C-12 (J = 1-0) and (CO)-C-13 (J = 1-0) datasets at 10 pc resolution, including a complete set of velocity channel distributions and comparisons with H and dust emissions as well as with the other broad molecular features. We have found new features on smaller scales in the loops, including helical distributions in the loop tops and vertical spurs. The loops have counterparts of the H I gas, indicating that the loops include atomic gas. The IRAS far-infrared emission is also associated with the loops, and was used to derive an X-factor of 0.7 (+/- 0.1) x 10(20) cm(-2) (K km s(-1))(-1) to convert the (CO)-C-12 intensity into the total molecular hydrogen column density. From the (CO)-C-12, (CO)-C-13, H I, and dust datasets we estimated the total mass of loops 1 and 2 to be similar to 1.4 x 10(6) M-circle dot and similar to 1.9 x 10(6) M-circle dot respectively, where the H I mass corresponds to similar to 10%-20% of the total mass and the total kinetic energy of the two loops is similar to 10(52) erg. An analysis of the kinematics of the loops yields that the loops are rotating at similar to 47 km s(-1) and expanding at similar to 141 km s(-1) at a radius of similar to 670 pc from the center. Fukui et al. (2006) presented a model that the loops are created by magnetic flotation due to the Parker instability with an estimated magnetic field strength of similar to 150 mu G. We present comparisons with the recent numerical simulations of the magnetized nuclear disk by Machida et al. (2009, PASJ, 61, 411) and Takahashi et al. (2009, PASJ, 61, 957), and show that the theoretical results are in good agreement with the observations. The helical distributions also suggest that some magnetic instability plays a role similarly to the solar helical features.

Fukui et al. (2006. Science, 314,106) discovered two molecular loops in the Galactic center, and argued that the foot points of the molecular loops, two bright spots at both loop ends, represent gas accumulated by the falling motion along the loops, subsequent to magnetic flotation by the Parker instability. We have carried out sensitive CO observations of the foot points toward l = 356 degrees at a few pc resolution in the six rotational transitions of CO: (CO)-C-12 (J = 1-0, 3-2, 4-3, 7-6), (CO)-C-13 (J = 1-0), and (CO)-O-18 (J = 1-0). A high-resolution image of (CO)-C-12 (J = 3-2) has revealed the detailed distribution of the high-excitation gas, including U shapes, the outer boundary of which shows sharp intensity jumps accompanying strong velocity gradients. An analysis of the multi-J CO transitions shows that the temperature is in the range from 30 to 100 K and the density is around 10(3)-10(4) cm(-3), confirming that the foot points have high temperature and density, although there is no prominent radiative heating source, such as high-mass stars in or around the loops. We argue that the high temperature is likely due to shock heating under the C-shock condition caused by magnetic flotation. We made a comparison of the gas distribution with theoretical numerical simulations, and note that the U shape is consistent with numerical simulations. We also find that the region of highest temperature of similar to 100 K or higher inside the U shape corresponds to the spur having an upward flow, additionally heated up either by magnetic reconnection or bouncing in the interaction with the narrow neck at the bottom of the U shape. We note that these new findings further reinforce the magnetic floatation interpretation.

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 beta = p(gas)/p(mag). We find magnetically supported (low-beta), 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-beta solutions extend to high mass accretion rates (greater than or similar to alpha(2)(M)over dot(Edd)) and the electron temperature is moderately cool (T(e) similar to 10(8)-10(9.5) K). High luminosities (greater than or similar to 0.1L(Edd)) and moderately high energy cutoffs in the X-ray spectrum (similar to 50-200 keV) observed in the bright/hard state can be explained by the low-beta solutions.

We determine the spin of a supermassive black hole in the context of disc-seismology by comparing newly detected quasi-periodic oscillations (QPOs) of radio emission in the Galactic centre, Sagittarius A* (Sgr A*), as well as infrared and X-ray emissions with those of the Galactic black holes. We find that the spin parameters of black holes in Sgr A* and in Galactic X-ray sources have a unique value of approximate to 0.44 which is smaller than the generally accepted value for supermassive black holes, suggesting evidence for the angular momentum extraction of black holes during the growth of supermassive black holes. Our results demonstrate that the spin parameter approaches the equilibrium value where spin-up via accretion is balanced by spin-down via the Blandford-Znajek mechanism regardless of its initial spin. We anticipate that measuring the spin of black holes by using QPOs will open a new window for exploring the evolution of black holes in the Universe.

We carried out two-dimensional magnetohydrodynamic simulations of the Galactic gas disk to show that the dense loop-like structures discovered by the Galactic center molecular cloud survey using the NANTEN 4-m telescope can be formed by a buoyant rise of magnetic loops due to the Parker instability. At the initial state, we assumed a gravitationally stratified disk consisting of a cool layer (T similar to 10(3) K), a warm layer (T similar to 10(4) K), and a hot layer (T similar to 10(5) K). The simulation box was a local part of the disk containing the equatorial plane. The gravitational field was approximated by that of a point mass at the Galactic center. The self-gravity, and the effects of the Galactic rotation were ignored. Numerical results indicate that the length of the magnetic loops emerging from the disk is determined by the scale height of the hot layer (similar to 100 pc at 1 kpc from the Galactic center). The loop length, velocity gradient along the loops, and large velocity dispersions at their foot points are consistent with the NANTEN observations. We also show that the loops become top-heavy when the curvature of the loop is sufficiently small, so that the rising loop accumulates the overlying gas faster than sliding it down along the loop. This mechanism is similar to that in the formation of solar chromospheric arch filaments. The molecular loops emerge from the low-temperature layer just like the dark filaments observed in the Ha image of the emerging flux region of the Sun.

We have discovered a molecular dome-like feature towards 355° ≤ l ≤ 359° and 0° ≤ b ≤ 2°. The large velocity dispersions of 50-100km s -1 of this feature are much larger than those in the Galactic disk, and indicate that the feature is located in the Galactic center, probably within ∼ 1 kpc of Sgr A * . The distribution has a projected length of ∼ 600 pc and a height of ∼ 300 pc from the Galactic disk, and shows a large-scale monotonic velocity gradient of ∼ 130 km s -1 per ∼ 600 pc. The feature is also associated with H I gas having a more continuous spatial and velocity distribution than that of 12 CO. We interpret the feature as being a magnetically floated loop similar to loops 1 and 2, and name it &quot;loop 3&quot;. Loop 3 is similar to loops 1 and 2 in its height and length, but is different from loops 1 and 2 in that the inner part of loop 3 is filled with molecular emission. We have identified two foot points at both ends of loop 3. H i, 12 CO, and 13 CO datasets were used to estimate the total mass and the kinetic energy of loop 3 to be ∼3.0 × 10 6 M Odot; and ∼ 1.7 × 10 52 erg. The huge size, velocity dispersions, and energy are consistent with the magnetic origin of the Parker instability, as in the case of loops 1 and 2, but is difficult to be explained by multiple stellar explosions. We argue that loop 3 is in an earlier evolutionary phase than loops 1 and 2 based on the innerfilled morphology and the relative weakness of the foot points. This discovery indicates that the western part of the nuclear gas disk of ∼ 1 kpc radius is dominated by the three well-developed magnetically floated loops, and suggests that the dynamics of the nuclear gas disk is strongly affected by magnetic instabilities. © 2009. Astronomical Society of Japan.

Supercritical accretion flows inevitably produce radiation-pressure driven outflows, which Compton up-scatter soft photons from the underlying accretion flow, thereby making hard emission. We performed two-dimensional radiation hydrodynamic simulations of supercritical accretion flows and outflows, while incorporating such Compton scattering effects, and demonstrated that there appears a new hard spectral state at higher photon luminosities than that of the slim-disk state. In this state, as the photon luminosity increases, the photon index decreases and the fraction of the hard emission increases. The Compton v-parameter is on the order of unity (and thus the photon index will be similar to 2) when the apparent photon luminosity is similar to 30 L-E (with L-E being the Eddington luminosity) for nearly face-on sources. This explains the observed spectral hardening of the ULX NGC 1313 X-2 in its brightening phase, and thus supports the model of supercritical accretion onto stellar-mass black holes in this ULX.

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 a warm (similar to 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-of-sight 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.

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/slowtransition 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 (beta = (p(gas) + p(rad))/p(mag) &lt; 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-beta disk (T similar to 10(7)-10(11)K) 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-beta branch extends to M(over dot) greater than or similar to 0.1 M(over dot)(Edd), where M(over dot) is the mass accretion rate and M(over dot)(Edd) is the Eddington mass accretion rate, in which the temperature anticorrelates with the mass accretion rate. Thus, optically thin low-beta disks can explain the bright hard state. Optically thick, low-beta disks have the radial dependence of the effective temperature T(eff) proportional to pi(-3/4). Such disks will be observed as staying in a high/soft state. Furthermore, limit cycle oscillations between an optically thick low-beta disk and a slim disk will occur because the optically thick low-beta 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.

We obtained self-similar solutions of relativistically expanding magnetic loops taking into account the azimuthal magnetic fields. We neglect stellar rotation and assume axisymmetry and a purely radial flow. As the magnetic loops expand, the initial dipole magnetic field is stretched into the radial direction. When the expansion speed approaches the light speed, the displacement current reduces the toroidal current and modifies the distribution of the plasma lifted up from the central star. Since these self-similar solutions describe the free expansion of the magnetic loops, i.e. dv/dt = 0, the equations of motion are similar to those of the static relativistic magnetohydrodynamics. This allows us to estimate the total energy stored in the magnetic loops by applying the virial theorem. This energy is comparable to that of the giant flares observed in magnetars.

We present the results of global 2D and 3D magnetohydrodynamic simulations of jet formation from a gas disk rotating around a central object. In a disk-star system, differential rotation twists magnetic loops connecting a star and its disk. As magnetic twist accumulates, the magnetic loops inflate and form current sheets inside the loops. Magnetic reconnection taking place in the current sheet can be the origin of X-ray flares observed in protostars. Numerical simulations using larger computing area revealed that the expanding magnetic loops form a magnetic tower. Magnetic reconnections taking place near the footpoints of the tower inject hot plasmoids into the tower. Less collimated outflow of cool gas emanates from the disk along the large-scale magnetic fields formed by the magnetic loop expansion. We also show that even when the large-scale poloidal magnetic fields do not exist at the initial state, they are generated by the buoyant rise of magnetic loops from the accretion disk. These magnetic loops are twisted, elongated, and form magnetic towers. Core-jet and outer wind structure is common both in AGNs and in protostars.

We carried out global three-dimensional resistive magnetohydrodynamic simulations of radiatively inefficient black hole accretion flows. The disk magnetic fields, amplified by MRI, create magnetic loops emerging from the disk. Subsequently, these loops are twisted by the differential rotation of the disk and form large-scale poloidal magnetic fields even when the initial magnetic field is purely azimuthal. Semi-relativistic outflows emerge along the large-scale poloidal magnetic fields. By analyzing the simulation results, we obtained 43-GHz image of the launching region of outflows. We also discuss the possibility of detecting dynamo-driven reversals of mean magnetic fields.

We present the results of global three-dimensional magneto-hydrodynamic simulations of black-hole accretion flows. We focus on the dependence of the numerical results on the gas temperature supplied from the outer region. General-relativistic effects were taken into account using the pseudo-Newtonian potential. We ignored radiative cooling of the accreting gas. The initial state was a torus threaded by a weak azimuthal magnetic field. We found that the mass-accretion rate and the mass-outflow rate strongly depend on the temperature of the initial torus. The ratio of the average Maxwell stress generated by the magneto-rotational instability (MRI) to the gas pressure, alpha equivalent to &lt; B omega B phi/4 pi &gt;/&lt; P &gt;, is alpha similar to 0.05 in a hot torus and alpha similar to 0.01 in a cool torus. In the cool model, a constant angular momentum inner torus is formed around 4-8 r(s), where r(s) is the Schwarzschild radius. This inner torus deforms itself from a circle to a crescent quasi-periodically. During this deformation, the mass-accretion rate, the magnetic energy and the Maxwell stress increase. As the magnetic energy is released, the inner torus returns to a circular shape and starts the next cycle. The power spectral density (PSD) of the time variation of the mass-accretion rate in the cool model has a low-frequency peak around 10 Hz when we assumed a 10M(circle dot) black hole. The mass outflow rate in the low temperature model also shows quasi-periodic oscillation.

We present the results of global three-dimensional magnetohydrodynamic simulations of galactic gas disks. Numerical results indicate that magnetorotational instability amplifies magnetic fields. The growth of magnetic fields saturates when B similar to 1.5 mu G. This saturation level is almost independent of the strength of the initial magnetic field. The magnetic loops buoyantly rising from the disk supply azimuthal magnetic flux to the halo. We found that mean azimuthal magnetic fields inside the disk reverse their direction with interval about 1Gyr. The field reversal takes place following the buoyant escape of azimuthal magnetic flux amplified in the disk. The quasi-periodic reversals of azimuthal magnetic fields create a magnetically striped halo, in which azimuthal magnetic fields reverse their direction with height.

We obtained steady solutions of magnetized black hole accretion disks taking into account the gas pressure, magnetic pressure, and radiation pressure.We assumed the bremsstrahlung cooling in the optically thin limit and the black body cooling in the optically thick limit.Wassumed that magnetic fields inside the disk are turbulent and dominated by azimuthal component and that the azimuthally averaged Maxwell stress is proportional to the total pressure. We found that when accretion rate exceeds the threshold for the onset of the thermal instability for optically thin disks, a magnetic pressure dominated, cool branch appears in the thermal equilibrium curve. This disk corresponds to the "Bright/Hard" state in black hole candidates observed during the transition from a Low/Hard state to a High/Soft state. © 2008 American Institute of Physics.

Steep gradients of temperature and density, called cold fronts, are observed by Chandra in a leading edge of subclusters moving through the intracluster medium ( ICM). The presence of cold fronts indicates that thermal conduction across the front is suppressed by magnetic fields. We carried out three- dimensional magnetohydrodynamic ( MHD) simulations including anisotropic thermal conduction of a subcluster moving through a magnetically turbulent ICM. We found that turbulent magnetic fields are stretched and amplified by shear flows along the interface between the subcluster and the ambient ICM. Since magnetic fields reduce the efficiency of thermal conduction across the front, the cold front survives for at least 1 Gyr. We also found that a moving subcluster works as an amplifier of magnetic fields. Numerical results indicate that stretched turbulent magnetic fields accumulate behind the subcluster and are further amplified by vortex motions. The moving subcluster creates a long tail of ordered magnetic fields, in which the magnetic field strength attains a value of beta= P-gas / P-mag &lt;= 10.

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 Maxwell stress is proportional to the total pressure. The radial advection rate of the azimuthal magnetic flux, Phi, is prescribed as being proportional to pi(-zeta), where pi is the radial coordinate and zeta is a parameter that parameterizes the radial variation of Phi. We found that when the accretion rate, M, exceeds the threshold for the onset of the thermal instability, a magnetic pressure-dominated new branch appears. Thus, the thermal-equilibrium curve of an optically thin disk has a 'Z'-shape in the plane of the 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/bard state to a high/soft state. We also obtained global steady transonic solutions containing such a transition layer.

We carried out 2D and 3D magnetohydrodynamic simulations including radiative cooling and anisotropic thermal conduction of a magnetically turbulent intracluster medium. In 3D simulations, we found that magnetic pressure increases by a factor of 2 at 1.4 Gyr because magnetic fields accumulate in the center along with the plasma contracting by the cooling instability. In 2D simulations including thermal conduction, we found that a low temperature region exists along the loop-shaped magnetic field lines. Thus, the restriction of thermal conduction across magnetic field lines enables the coexistence of hot and cool plasmas in cluster cores.