arXiv subjects

Shinji Koide

Publications and source records attributed to Shinji Koide.

At least 19 recordsLinked to original sources

arXiv2025

One-dimensional linear analysis and numerical simulations of Alfven waves in a force-free magnetosphere around a Kerr black hole

We perform one-dimensional linear analysis and numerical simulations of the propagation of Alfven waves in a force-free magnetosphere along magnetic field lines around a spinning black hole. We use the results to investigate the dynamic process of wave propagation and energy transport for Alfven waves. As in a previous study using the Banados--Teitelboim--Zanelli spacetime (Koide et al. 2022), the Alfven wave induces a fast magnetosonic wave in the case of a spinning black hole. Energy conservation is confirmed when this additional induced magnetosonic wave is considered. We also observe the reflection of the inwardly propagating Alfven wave around the static limit, which is prohibited in theory when using the eikonal approximation.

astro-ph.HE↗

arXiv2023

A relativistic two-stream instability in an extremely low-density plasma

A linear analysis based on two-fluid equations in the approximation of a cold plasma, wherein the plasma temperature is assumed to be zero, demonstrates that a two-stream instability occurs in all cases. However, if this were true, the drift motion of electrons in an electric current over a wire would become unstable, inducing an oscillation in an electric circuit with ions bounded around specific positions. To avoid this peculiar outcome, we must assume a warm plasma with a finite temperature when discussing the criterion of instability. The two-stream instability in warm plasmas has typically been analyzed using kinetic theory to provide a general formula for the instability criterion from the distribution function of the plasma. However, the criteria based on kinetic theory do not have an easily applicable form. Here, we provide an easily applicable criterion for the instability based on the two-fluid model at finite temperatures, extensionally in the framework of special relativity. This criterion is relevant for analyzing two-stream instabilities in low-density plasmas in the universe and in Earth-based experimental devices.

physics.plasm-ph↗

arXiv2021

One-dimensional force-free numerical simulations of Alfven waves around a spinning black string

We performed one-dimensional force-free magnetodynamic numerical simulations of the propagation of Alfven waves along magnetic field lines around a spinning black-hole-like object, the Banados--Teitelboim--Zanelli black string, to investigate the dynamic process of wave propagation and energy transport with Alfven waves. We considered axisymmetric and stationary magnetosphere and perturbed the background magnetosphere to obtain the linear wave equation for the Alfven wave mode. The numerical results show that the energy of Alfven waves monotonically increases as the waves propagate outwardly along the rotating curved magnetic field line around the ergosphere, where energy seems not to be conserved, in the case of energy extraction from the black string by the Blandford--Znajek mechanism. The apparent breakdown of energy conservation suggests the existence of an additional wave induced by the Alfven wave. Considering the additional fast magnetosonic wave induced by the Alfven wave, the energy conservation is recovered. Similar relativistic phenomena, such as the amplification of Alfven waves and induction of fast magnetosonic waves, are expected around a spinning black hole.

astro-ph.HE↗

arXiv2020

Generalized general-relativistic magnetohydrodynamic equations for plasmas of active galactic nuclei in the era of the Event Horizon Telescope

The generalized general-relativisticmagnetohydrodynamic (generalized GRMHD) equations have been used to study specific relativistic-plasmas phenomena, such as relativistic magnetic reconnection or wave propagation modified by non-ideal MHD effects. However, the "Theta" term in the generalized Ohm law, which expresses the energy exchange between two fluids composing a plasma, has yet to be determined in these equations. In this paper, we determine the "Theta" term based on the generalized relativistic Ohm law itself. This provides closure of the generalized GRMHD equations, yielding a closed system of the equations of relativistic plasma. According to this system of equations, we reveal the characteristic scales of non-ideal MHD phenomena and clarify the applicable condition of the ideal GRMHD equations. We evaluate the characteristic scales of the non-ideal MHD phenomena in the M87* plasma using the Event Horizon Telescope observational data.

astro-ph.HE↗

arXiv2019

Analytical and Numerical Methods and Test Calculations of One-Dimensional Force--Free Magnetodynamics on Arbitrary Magnetic Surfaces across Horizons of Spinning Black Holes

Numerical simulations of the force-free magnetodynamics (FFMD) of the electromagnetic field around a spinning black hole are useful to investigate the dynamic electromagnetic processes around a spinning black hole, such as the emergence of the Blandford--Znajek mechanism. To reveal the basic physics of magnetic fields around a black hole through the dynamic process, we use one-dimensional (1D) FFMD along the axisymmetric magnetic surface, which provides a relatively simple, sufficiently precise, and powerful tool to analyze the dynamic process around a spinning black hole. We review the analytic and numerical aspects of 1D FFMD for an arbitrary magnetic surface around a black hole. In addition, we also show some numerical simulation test results for three types of magnetic surfaces at the equatorial plane of the black hole.

astro-ph.HE↗

arXiv2019

Numerical simulation of magnetic reconnection around a black hole

We performed numerical simulations of general relativistic magnetohydrodynamics with uniform resistivity to investigatethe occurrence of magnetic reconnection in a split-monopole magnetic field around a Schwarzschild black hole. We found that magnetic reconnection happens near the black hole at its equatorial plane. The magnetic reconnection has a point-like reconnection region and slow shock waves, as in the Petschek reconnection model. The magnetic reconnection rate decreases as the resistivity becomes smaller. When the global magnetic Reynolds number is $10^4$ or larger, the magnetic reconnection rate increases linearly with time from $2 \tau_{\rm S}$ to $\sim 10 \tau_{\rm S}$ ($\tau_{\rm S}=r_{\rm S}/c, r_{\rm S}$ is the Schwarzschild radius and $c$ is the speed of light). The linear increase of the reconnection rate agrees with the magnetic reconnection in the Rutherford regime of the tearing mode instability.

astro-ph.HE↗

arXiv2014

Causal extraction of black hole rotational energy by various kinds of electromagnetic fields

Recent general relativistic MHD simulations have suggested that relativistic jets from active galactic nuclei (AGNs) have been powered by rotational energy of central black holes. Some mechanisms of extraction of black hole rotational energy have been proposed, like the Penrose process, Blandford-Znajek mechanism, MHD Penrose process, and superradiance. The Blandford-Znajek mechanism is the most promising mechanism for the engines of the relativistic jets from AGNs. However, an intuitive interpretation of this mechanism with causality is not clarified yet, while the Penrose process has a clear interpretation for the causal energy extraction from the black hole with negative energy. In this paper, we present a formula to build physical intuition so that in the Blandford-Znajek mechanism as well as other electromagnetic processes, negative electromagnetic energy plays an important role to extract the rotational energy of the black holes causally.

astro-ph.HE↗

arXiv2011

Group velocity and causality in standard relativistic resistive magnetohydrodynamics

Group velocity of electromagnetic waves in plasmas derived by standard relativistic resistive MHD (resistive RMHD) equations is superluminal. If we assume that the group velocity represents the propagation velocity of a signal, we have to worry about the causality problem. That is, some acausal phenomena may be induced, such that information transportation to the absolute past and spontaneous decrease in the entropy. Here, we tried to find the acausal phenomena using standard resistive RMHD numerical simulations in the suggested situation of the acausal phenomena. The calculation results showed that even in such situations no acausal effect happens. The numerical result with respect to the velocity limit of the information transportation is consistent with a linear theory of wave train propagation. Our results assure that we can use these equations without problems of acausal phenomena.

astro-ph.HE↗

arXiv2010

Charge separation instability in an unmagnetized disk plasma around a Kerr black hole

In almost all of plasma theories for astrophysical objects, we have assumed the charge quasi-neutrality of unmagnetized plasmas in global scales. This assumption has been justified because if there is a charged plasma, it induces electric field which attracts the opposite charge, and this opposite charge reduces the charge separation. Here, we report a newly discovered instability which causes a charge separation in a rotating plasma inside of an innermost stable circular orbit (ISCO) around a black hole. The growth rate of the instability is smaller than that of the disk instability even in the unstable disk region and is forbidden in the stable disk region outside of the ISCO. However, this growth rate becomes comparable to that of the disk instability when the plasma density is much lower than a critical density inside of the ISCO. In such case, the charge separation instability would become apparent and cause the charged accretion into the black hole, thus charge the hole up.

astro-ph.HE↗

arXiv2009

Generalized General Relativistic MHD Equations and Distinctive Plasma Dynamics around Rotating Black Holes

To study phenomena of plasmas around rotating black holes, we have derived a set of 3+1 formalism of generalized general relativistic magnetohydrodynamic (GRMHD) equations. Especially, we investigated general relativistic phenomena with respect to the Ohm's law. We confirmed the electromotive force due to the gravitation, centrifugal force, and frame-dragging effect in plasmas near the black holes. These effects are significant only in the local small-scale phenomena compared to the scale of astrophysical objects. We discuss the possibility of magnetic reconnection, which is triggered by one of these effects in a small-scale region and influences the plasmas globally. We clarify the conditions of applicability of the generalized GRMHD, standard resistive GRMHD, and ideal GRMHD for plasmas in black hole magnetospheres.

astro-ph.HE↗

arXiv2009

Generalized Relativistic Magnetohydrodynamic Equations for Pair and Electron-Ion Plasmas

We derived one-fluid equations based on a relativistic two-fluid approximation of e$^\pm$ pair plasma and electron-ion plasma to reveal the specific relativistic nature of their behavior. Assuming simple condition on the relativistic one-fluid equations, we propose generalized relativistic magnetohydrodynamic (RMHD) equations which satisfy causality. We show the linear analyses of these equations regarding various plasma waves to show the validity of the generalized RMHD equations derived here and to reveal the distinct properties of the pair plasma and electron-ion plasma. The distinct properties relate to (i) the inertia effect of electric charge, (ii) the momentum of electric current, (iii) the relativistic Hall effect, (iv) the thermal electromotive force, and (v) the thermalized energy exchange between the two fluids. Using the generalized RMHD equations, we also clarify the condition that we can use standard RMHD equations and that we need the distinct RMHD equations of pair and electron-ion plasmas. The standard RMHD is available only when the relative velocity of the two fluids is nonrelativistic, a difference of the enthalpy densities of the two fluids is much smaller than the total enthalpy density, and the above distinct properties of the pair/electron-ion plasma are negligible. We discuss a general relativistic version of the equations applicable to the pair and electron-ion plasmas in black hole magnetospheres. We find the effective resistivity due to shear of frame ragging around a rotating black hole.

astro-ph.HE↗

arXiv2008

Propagation of Electromagnetic Waves in Resistive Pair Plasma and Causal Relativistic Magnetohydrodynamics

We investigate the propagation of electromagnetic waves in resistive pair plasmas using a onefluid theory derived from the relativistic two-fluid equations. When the resistivity normalized by the electron/positron inertia variable exceeds a critical value, the dispersion relation for electromagnetic waves shows that the group velocity is larger than the light speed in vacuum. However, in such a case, it also is found that the plasma parameter is less than unity: that is, the electron-positron pair medium no longer can be treated as plasma. Thus the simple two-fluid approximation is invalid. This confirms that superluminal propagation of electromagnetic wave is forbidden in a plasma -- a conclusion consistent with the relativistic principle of causality. As an alternative, we propose a new set of equations for ``causal relativistic magnetohydrodynamics", which both have non-zero resistivity and yet are consistent with the causality principle.

physics.plasm-ph↗

arXiv2008

Energy Extraction from a Rotating Black Hole by Magnetic Reconnection in Ergosphere

We investigate mechanisms of energy extraction from a rotating black hole in terms of negative energy-at-infinity. In addition to the Penrose process through particle fission, the Blandford-Znajek mechanism by magnetic tension, and the magnetohydrodynamic Penrose process, we examine energy extraction from a black hole caused by magnetic reconnection in the ergosphere. The reconnection redistributes the angular momentum efficiently to yield the negative energy-atinfinity. We derive a condition for the process to operate in a simple situation, where the plasma is incompressible and the magnetic energy is converted completely to the plasma kinetic energy locally. Astrophysical situations of magnetic reconnection around the black holes are also discussed.

astro-ph↗

arXiv2006

General Relativistic Magnetohydrodynamic Simulations of Jet Formation with a Thin Keplerian Disk

We have performed several simulations of black hole systems (non-rotating, black hole spin parameter a=0.0 and rapidly rotating, a=0.95) with a geometrically thin Keplerian disk using the newly developed RAISHIN code. The simulation results show the formation of jets driven by the Lorentz force and the gas pressure gradient. The jets have mildly relativistic speed (\sim 0.4c). The matter is continuously supplied from the accretion disk and the jet propagates outward until each applicable terminal simulation time (non-rotating: t/τ_S = 275 and rotating: t/tau_S = 200, τ_S \equiv r_S/c). It appears that a rotating black hole creates an additional, faster, and more collimated matter-dominated inner outflow (\sim 0.5c) formed and accelerated by the twisted magnetic field resulting from frame-dragging in the black hole ergosphere. This is the first known simulation confirming the formation of an inner magnetically-driven, matter-dominated jet by the frame-dragging effect from a black hole co-rotating with a thin Keplerian disk threaded by a vertical magnetic field. This result indicates that jet kinematic structure depends on black hole rotation and on the initial magnetic field configuration and strength.

astro-ph↗

arXiv2006

RAISHIN: A High-Resolution Three-Dimensional General Relativistic Magnetohydrodynamics Code

We have developed a new three-dimensional general relativistic magnetohydrodynamic (GRMHD) code, RAISHIN, using a conservative, high resolution shock-capturing scheme. The numerical fluxes are calculated using the Harten, Lax, & van Leer (HLL) approximate Riemann solver scheme. The flux-interpolated, constrained transport scheme is used to maintain a divergence-free magnetic field. In order to examine the numerical accuracy and the numerical efficiency, the code uses four different reconstruction methods: piecewise linear methods with Minmod and MC slope-limiter function, convex essentially non-oscillatory (CENO) method, and piecewise parabolic method (PPM) using multistep TVD Runge-Kutta time advance methods with second and third-order time accuracy. We describe code performance on an extensive set of test problems in both special and general relativity. Our new GRMHD code has proven to be accurate in second order and has successfully passed with all tests performed, including highly relativistic and magnetized cases in both special and general relativity.

astro-ph↗

arXiv2004

General Relativistic Magnetohydrodynamic Simulations of Collapsars: Rotating Black Hole Cases

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of collapsars including a rotating black hole. This paper is an extension of our previous paper (Mizuno et al. 2004). The current calculation focuses on the effect of black hole rotation using general relativistic MHD with simplified microphysics. Initially, we assume that the core collapse is failed in this star. A few solar mass rotating black hole is inserted by hand into the calculation. We consider two cases, the co-rotating case and counter-rotating case with respect to the black hole rotation. Although the counter-rotating case may be unrealistic for collapsar, we perform as a maximally dragging case of magnetic field. The simulation results show the formation of a disk-like structure and the generation of a jet-like outflow near the central black hole. The jet-like outflow propagates outwardly with the twisted magnetic field and becomes collimated. We have found that the jets are generated and accelerated mainly by the magnetic field. The total jet velocity in the rotating black hole case is comparable to that of the non-rotating black hole case (Mizuno et al. 2004), 0.3c. When the rotation of the black hole is faster, the magnetic field is twisted strongly owing to the frame-dragging effect. The magnetic energy stored by the twisting magnetic field is converted to kinetic energy of the jet directly rather than propagating as an Alfven wave. Thus, as the rotation of the black hole becomes faster, the poloidal velocity of the jet becomes faster. In the rapidly rotating black hole case the jet-like outflow can be produced by the frame dragging effect only through the twisting of magnetic field even if there is no stellar rotation.

astro-ph↗

arXiv2004

General Relativistic Magnetohydrodynamic Simulations of Collapsars

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of the gravitational collapse of a magnetized rotating massive star as a model of gamma ray bursts (GRBs). The current calculation focuses on general relativistic MHD with simplified microphysics (we ignore neutrino cooling, physical equation of state, and photodisintegration). Initially, we assume that the core collapse has failed in this star. A few $M_{\odot}$ black hole is inserted by hand into the calculation. The simulations presented in the paper follow the accretion of gas into a black hole that is assumed to have formed before the calculation begins.The simulation results show the formation of a disk-like structure and the generation of a jetlike outflow inside the shock wave launched at the core bounce. We have found that the jet is accelerated by the magnetic pressure and the centrifugal force and is collimated by the pinching force of the toroidal magnetic field amplified by the rotation and the effect of geometry of the poloidal magnetic field. The maximum velocity of the jet is mildly relativistic (0.3c). The velocity of the jet becomes larger as the initial rotational velocity of stellar matter gets faster. On the other hand, the dependence on the initial magnetic field strength is a bit more complicated: the velocity of the jet increases with the initial field strength in the weak field regime, then is saturated at some intermediate field strength, and decreases beyond the critical field strength. These results are related to the stored magnetic energy determined by the balance between the propagation time of the Alfven wave and the rotation time of the disk (or twisting time).

astro-ph↗

arXiv2003

General Relativistic MHD Simulations of the Gravitational Collapse of a Rotating Star with Magnetic Field as a Model of Gamma-Ray Bursts

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of the gravitational collapse of a magnetized rotating massive star as a model of gamma ray bursts (GRBs). This simulation showed the formation of a disk-like structure and the generation of a jet-like outflow inside the shock wave launched at the core bounce. We have found the jet is accelerated by the magnetic pressure and the centrifugal force and is collimated by the pinching force of the toroidal magnetic field amplified by the rotation and the effect of geometry of the poloidal magnetic field. The maximum velocity of the jet is mildly relativistic ($\sim$ 0.3 c).

astro-ph↗