Publications (12)47.75 Total impact

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ABSTRACT: By performing 2.5dimensional special relativistic radiation magnetohydrodynamics simulations, we study the supercritical accretion disks and the outflows launched via the radiation force. We find that the outflow is accelerated by the radiation flux force, but the radiation drag force prevents the outflow velocity from increasing. The outflow velocity saturates around 3040% of the light speed around the rotation axis, since then the flux force balances with the drag force. Our simulations show that the outflow velocity is kept nearly constant in the regime of \dot{M}_{BH} ~ 1001000 L_{Edd}/c^2, where \dot{M}_{BH} is the mass accretion rate, L_{Edd} is the Eddington luminosity, and c is the light speed. Such a faster outflow is surrounded by a slower outflow of ~ 0.1c. This velocity is also determined by force balance between the radiation flux force and the radiation drag. The radiation drag works to collimate the slower outflow in cooperation with the Lorentz force, although the faster outflow is mainly collimated by the Lorentz force. The kinetic energy is carried by the slower outflow rather than by the faster outflow. The total kinetic luminosity of the outflow as well as the photon luminosity is ~ L_{Edd}, almost independent of the mass accretion rate. 
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ABSTRACT: We investigate effects of cosmicrays on the linear growth of the KelvinHelmholtz instability. Cosmicrays are treated as an adiabatic gas and allowed to diffuse along magnetic field lines. We calculated the dispersion relation of the instability for various sets of two free parameters, the ratio of the cosmicray pressure to the thermal gas pressure and the diffusion coefficient. Including cosmicray effects, a shear layer is more destabilized and the growth rates can be enhanced in comparison with the ideal magnetohydrodynamical case. Whether the growth rate is effectively enhanced or not depends on the diffusion coefficient of cosmicrays. We obtain the criterion for effective enhancement by comparing the growing time scale of the instability with the diffusion time scale of cosmicrays. These results can be applied to various astrophysical phenomena where a velocity shear is present, such as outflows from starforming galaxies, AGN jet, channel flows resulting from the nonlinear development of the magnetorotational instability, and galactic disks.The Astrophysical Journal 04/2014; 787(2). DOI:10.1088/0004637X/787/2/169 · 6.28 Impact Factor 
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ABSTRACT: We develop a numerical scheme for solving a fully special relativistic resistive radiation magnetohydrodynamics. Our code guarantees conservations of total mass, momentum and energy. Radiation energy density and radiation flux are consistently updated using the M1 closure method, which can resolve an anisotropic radiation fields in contrast to the Eddington approximation as well as the fluxlimited diffusion approximation. For the resistive part, we adopt a simple form of the Ohm's law. The advection terms are explicitly solved with an approximate Riemann solver, mainly HLL scheme, and HLLC and HLLD schemes for some tests. The source terms, which describe the gasradiation interaction and the magnetic energy dissipation, are implicitly integrated, relaxing the CourantFriedrichsLewy condition even in optically thick regime or a large magnetic Reynolds number regime. Although we need to invert $4\times 4$ (for gasradiation interaction) and $3\times 3$ (for magnetic energy dissipation) matrices at each grid point for implicit integration, they are obtained analytically without preventing massive parallel computing. We show that our code gives reasonable outcomes in numerical tests for ideal magnetohydrodynamics, propagating radiation, and radiation hydrodynamics. We also applied our resistive code to the relativistic Petschek type magnetic reconnection, revealing the reduction of the reconnection rate via the radiation drag.The Astrophysical Journal 05/2013; 772(2). DOI:10.1088/0004637X/772/2/127 · 6.28 Impact Factor 
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ABSTRACT: We propose an explicitimplicit scheme for numerically solving Special Relativistic Radiation Hydrodynamic (RRHD) equations, which ensures a conservation of total energy and momentum (matter and radiation). In our scheme, 0th and 1st moment equations of the radiation transfer equation are numerically solved without employing a fluxlimited diffusion (FLD) approximation. For an hyperbolic term, of which the time scale is the light crossing time when the flow velocity is comparable to the speed of light, is explicitly solved using an approximate Riemann solver. Source terms describing an exchange of energy and momentum between the matter and the radiation via the gasradiation interaction are implicitly integrated using an iteration method. The implicit scheme allows us to relax the CourantFriedrichsLewy condition in optically thick media, where heating/cooling and scattering timescales could be much shorter than the dynamical timescale. We show that our numerical code can pass test problems of one and twodimensional radiation energy transport, and onedimensional radiation hydrodynamics. Our newly developed scheme could be useful for a number of relativistic astrophysical problems. We also discuss how to extend our explicitimplicit scheme to the relativistic radiation magnetohydrodynamics.The Astrophysical Journal 12/2012; 764(2). DOI:10.1088/0004637X/764/2/122 · 6.28 Impact Factor 
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ABSTRACT: The efficiency of the energy conversion rate in the relativistic magnetic reconnection is investigated by means of Relativistic Resistive Magnetohydrodynamic (R2MHD) simulations. We confirmed that the simple SweetParker type magnetic reconnection is a slow process for the energy conversion as theoretically predicted by Lyubarsky (2005). After the SweetParker regime, we found a growth of the secondary tearing instability in the elongated current sheet. Then the energy conversion rate and the outflow velocity of reconnection jet increase rapidly. Such a rapid energy conversion would explain the time variations observed in many astrophysical flaring events. To construct a more realistic model of relativistic reconnection, we extend our R2MHD code to R3MHD code by including the radiation effects (Relativistic Resistive Radiation Magnetohydrodynamics R3MHD). The radiation field is described by the 0th and 1st moments of the radiation intensity (Farris et al. 2008, Shibata et al. 2011). The code has already passed some onedimensional and multidimensional numerical problems. We demonstrate the first results of magnetic reconnection in the radiation dominated current sheet.Proceedings of the International Astronomical Union 09/2012; 7(S279):405406. DOI:10.1017/S1743921312013567 
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ABSTRACT: Relativistic SweetParker type magnetic reconnection is investigated by relativistic resistive magnetohydrodynamic (RRMHD) simulations. As an initial setting, we assume antiparallel magnetic fields and a spatially uniform resistivity. A perturbation imposed on the magnetic fields triggers magnetic reconnection around a current sheet, and the plasma inflows into the reconnection region. The inflows are then heated due to ohmic dissipation in the diffusion region, and finally become relativistically hot outflows. The outflows are not accelerated to ultrarelativistic speeds (i.e., Lorentz factor ~ 1), even when the magnetic energy dominates the thermal and rest mass energies in the inflow region. Most of the magnetic energy in the inflow region is converted into the thermal energy of the outflow during the reconnection process. The energy conversion from magnetic to thermal energy in the diffusion region results in an increase in the plasma inertia. This prevents the outflows from being accelerated to ultrarelativistic speeds. We find that the reconnection rate R obeys the scaling relation R S^{0.5}, where S is the Lundquist number. This feature is the same as that of nonrelativistic reconnection. Our results are consistent with the theoretical predictions of Lyubarsky (2005) for SweetParker type magnetic reconnection.The Astrophysical Journal Letters 08/2011; 739(2). DOI:10.1088/20418205/739/2/L53 · 5.60 Impact Factor 
Article: Magnetic Field Decay Due to the Waveparticle Resonances in the Outer Crust of Neutron Stars
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ABSTRACT: Bearing in mind the application to the outer crust of neutron stars (NSs), we investigate the magnetic field decay by means of the fully relativistic ParticleInCell simulations. Numerical computations are carried out in two dimensions, in which the initial magnetic fields are set to be composed both of the uniform magnetic fields that model the global fields penetrating the NS and of the turbulent magnetic fields that would originate from the Hall cascade of the largescale turbulence. Our results show that the whistler cascade of the turbulence transports the magnetic energy preferentially in the direction perpendicular to the uniform magnetic fields. It is also found that the distribution function of electrons becomes anisotropic because electrons with lower energies are predominantly heated in the direction parallel to the uniform magnetic fields due to the Landau resonance, while electrons with higher energies are heated mainly by the cyclotron resonance that makes the distribution function isotropic for the high energy tails. Furthermore, we point out that the degree of anisotropy takes on the maximum value as a function of the initial turbulent magnetic energy. As an alternative to the conventional Ohmic dissipation, we propose that the magnetic fields in the outer crust of NSs, cascading down to the electron inertial scale via the whistler turbulence, would decay predominantly by the dissipation processes through the Landau damping and the cyclotron resonance.The Astrophysical Journal 02/2011; 728. DOI:10.1088/0004637X/728/2/151 · 6.28 Impact Factor 
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ABSTRACT: We obtained selfsimilar 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 (Takahashi, Asano, & Matsumoto 2009) 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 selfsimilar 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 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 \propto r^{\delta}. We alter the power law index \delta from 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 \delta is larger than 3, while it decreases with time when \delta is smaller than 3. The shock Lorentz factor is proportional to t^{(\delta3)/2}. These results are consistent with the analytical studies by Shapiro (1979).Monthly Notices of the Royal Astronomical Society 02/2011; 414. DOI:10.1111/j.13652966.2011.18520.x · 5.23 Impact Factor 
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ABSTRACT: Based on the characteristics of the magnetorotational instability (MRI) and the MRIdriven turbulence, we construct a steady model for a geometrically thin disk using "nonstandard" $\alpha$prescription. The efficiency of the angular momentum transport depends on the magnetic Prandtl number, $Pm = \nu/\eta$, where $\nu$ and $\eta$ are the microscopic viscous and magnetic diffusivities. In our disk model, ShakuraSunyaev's $\alpha$parameter has a powerlaw dependence on the magnetic Prandtl number, that is $\alpha \propto Pm^\delta$ where $\delta$ is the constant powerlaw index. Adopting Spitzer's microscopic diffusivities, the magnetic Prandtl number becomes a decreasing function of the disk radius when $\delta > 0$. The transport efficiency of the angular momentum and the viscous heating rate are thus smaller in the outer part of the disk, while these are impacted by the size of index $\delta$. We find that the disk becomes more unstable to the gravitational instability for a larger value of index $\delta$. The most remarkable feature of our disk model is that the thermal and secular instabilities can grow in its middle part even if the radiation pressure is negligibly small in the condition $\delta > 2/3$. In the realistic disk systems, it would be difficult to maintain the steady mass accretion state unless the $Pm$dependence of MRIdriven turbulence is relatively weak. Comment: 9 pages, 6 figures, Accepted for publication in ApJThe Astrophysical Journal 11/2010; 727(2). DOI:10.1088/0004637X/727/2/106 · 6.28 Impact Factor 
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ABSTRACT: We investigate the dynamics of the relativistic Sweet‐Parker reconnection by means of Relativistic Resistive Magnetohydrodynamic (RRMHD) simulation. It is found that the magnetic energy is preferentially converted to the thermal energy in the diffusion region, resulting the formation of the relativistically hot outflows. The outflow speed is a little smaller than that expected from the non‐relativistic theory which predicts the outflow speed is of order the Alfvén speed. The slow outflow speed is the consequence of the increase in the plasma inertia due to the relativistic effects. Since the observed Lorentz factor of the outflow and the reconnection rate are both small, our numerical model suggests that the relativistic Sweet‐Parker reconnection is a slow process for the magnetic energy conversion as well as the non‐relativistic one.10/2010; 1279(1):427429. DOI:10.1063/1.3509335 
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ABSTRACT: We obtained selfsimilar 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 selfsimilar 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.Monthly Notices of the Royal Astronomical Society 03/2009; 394(1):547  568. DOI:10.1111/j.13652966.2008.14357.x · 5.52 Impact Factor 
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ABSTRACT: We study the magnetic reconnection for relativistic plasma in the framework of magnetohydrody namics and by means of the ParticleInCell (PIC) simulation. In the former approach, the magnetic fields are assumed to reconnect steadily inside the diffusion region as assumed in the conventional SweetParker model. The model takes account of the pressure gradient between the diffusion and out flow regions as well as increase in the inertia due to the thermal energy. At a given inflow velocity (reconnection rate), the outflow velocity is faster when the Poynting to plasma kinetic fluxes in the in flow region (namely the magnetization parameter σi) is larger. For σi � 1, the plasma outflow velocity decreases with the increase in the inflow velocity since the larger heating rate enhances the inertia. We also carried out 2dimensional PIC simulations to study the σidependence of the outflow speed. The results indicate that the outflow velocity converges to a saturation value in the limit of large σi. On the other hand, the thermal energy increases with σi without saturation. These results are consistent with those based on the MHD analysis.
Publication Stats
35  Citations  
47.75  Total Impact Points  
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Institutions

2014

Kyoto University
Kioto, Kyōto, Japan


2011–2013

National Astronomical Observatory of Japan
 Center for Computational Astrophysics
Edo, Tōkyō, Japan


2009

Chiba University
 Graduate School of Science and Technology
Chibashi, Chibaken, Japan
