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-supported state (low/hard state) to a cooler, 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.

We studied the global structures of radiatively inefficient black hole accretion flows by three-dimensional magnetohydrodynamic simulations. The initial state is a constant angular-momentum torus threaded by weak toroidal magnetic fields. The torus is assumed to be embedded in a low-density, spherical, isothermal halo. We found that in the innermost region ($\varpi < 15 \,r_g$: $r_g$ is the Schwarzschild radius) of optically thin hot disks formed by accretion from the torus, the radial structure can be described by a one-dimensional steady transonic ADAF (Advection Dominated Accretion Flow) solution that takes into account the radial dependence of $\alpha$. Numerical results indicate that the magnetic flux emerging from the disk creates a magnetically active layer in which magnetic reconnection generates outflows along magnetic field lines. We also studied the dependence of the disk structure on the external pressure of the initial torus. The saturation level of magnetic field amplification is higher for a model with a larger external pressure. Thus, external pressure enhances the magnetic activity of the disk. When the external pressure is 5–20% of the maximum pressure of the torus, the kinetic and Poynting energy flux of the outflow is about 1% of the energy flux advected to the black hole through accretion.

In astrophysical jets observed in active galactic nuclei and in microquasars, the energy exchange rate by Coulomb collision is insufficient for thermal equilibrium between ions and electrons. Therefore, it is necessary to consider the difference between the ion temperature and the electron temperature. We present the results of two-temperature magnetohydrodynamics(MHD) simulations to demonstrate the effects of Coulomb coupling. It is assumed that the thermal dissipation heats only ions. We find that the ion and electron temperatures are separated through shocks. Since the ion entropy is increased by energy dissipation at shocks and the Coulomb collisions are inefficient, electron temperature becomes about 10 times lower than the ion temperature in the hotspot ahead of the jet terminal shock. In the cocoon, electron temperature decreases by gas mixing between high temperature cocoon gas and low temperature shocked-ambient gas even when we neglect radiative cooling, but electrons can be heated through collisions with ions. Radiation intensity maps are produced by post processing numerical results. Distributions of the thermal bremsstrahlung radiation computed from electron temperature have bright filament and cavity around the jet terminal shock.

Thermal conduction plays an important role in bimodal accretion flows consisting of high-temperature flow and cool flow, especially when the temperature is high and/or has a steep gradient. For example, in hard-to-soft transitions of black hole accretion flows, thermal conduction between the high-temperature region and the low-temperature region is appropriately considered. We conducted two-dimensional magnetohydrodynamic (MHD) numerical simulations considering anisotropic heat conduction to study condensation of geometrically thick hot accretion flows driven by radiative cooling during state transitions. Numerical results show that the intermediate region appears between the hot corona and the cool accretion disk when we consider heat conduction. The typical temperature and number density of the intermediate region of the 10 $M_{\odot }$ black hole at $10 R_{\mathrm{g}}$ ( $R_{\mathrm{g}}=3.0\times {10}^6\mathrm{cm}$ is the Schwarzschild radius) are $4\times {10}^{10}<T\hspace{0.17em}[\mathrm{K}]<4\times {10}^{12}$ and $5\times {10}^{15}<n\hspace{0.17em}[{\mathrm{cm}}^{-3}]<5\times {10}^{17}$ , respectively. The thickness of intermediate region is about half of the radius. By comparing two models with or without thermal conduction, we demonstrate the effects of thermal conduction.

Magnetic fields in the universe play an essential role in observations of the radio synchrotron continuum; however, we do not know enough about them, either observationally or theoretically. We are interested in galactic magnetic fields because they affect the structural formation of galaxies in terms of star-forming regions, spiral arms, and threads at the galactic center. To clarify the importance of magnetic fields, we carried out numerical simulations of the galactic gaseous disk with magnetic fields. We also calculated observables, such as the rotation measure and Stokes parameters, from the results of numerical simulation. FD maps and intensity maps have been reported, and the relation between azimuthal angle and FD has been shown to depend on the inclination of the observer. Furthermore, it has been shown that a polarized intensity below 800 MHz reflects field structure in the halo region, although the intensity is weak. The present paper summarizes the effects of Faraday depolarization and the relation between magnetic-field structure and Stokes parameters.

Tight and compact binary systems, such as double neutron star binaries, are believed to undergo a common envelope evolution phase, resulting in strongly bound orbits. During this phase, the outer layers of the primary star are expelled, resulting in orbital shrinkage. However, a part of the expelled material may remain as a circumbinary disk, which can further influence subsequent orbital evolution. In this study, we investigated orbital evolution in the presence of a circumbinary disk within a simplified framework by assuming that orbital contraction and disk dissipation occur over the viscous timescale. The results showed that the orbit of the binary system after the common envelope evolution phase was further contracted by up to ${\sim }17\%$ due to the presence of the circumbinary disk, irrespective of the disk’s mass and structure. This additional orbital contraction following the common envelope evolution phase may have significant implications for the formation rate of double neutron star binaries that merge within a cosmic timescale.