arXiv subjects

Toru Okuda

Publications and source records attributed to Toru Okuda.

14 recordsLinked to original sources

arXiv2026

Three-dimensional Global Relativistic Radiation Magnetohydrodynamics of Magnetically Arrested Disk Accretion Flows in AGNs

We perform three-dimensional radiation-relativistic magnetohydrodynamic (3D Rad-RMHD) simulations of accretion flows around spinning active galactic nuclei (AGNs). Our study focuses on the magnetically arrested disk (MAD) state, adopting a single-temperature model that includes bremsstrahlung opacity as the sole radiation process while varying the black hole spin from non-spinning to rapidly spinning cases. We find that the MAD state persists across all spin values, as demonstrated by the normalized magnetic flux at the horizon and the physically motivated spatially averaged plasma beta. The overall flow dynamics remain qualitatively similar for all spin models in 3D flow, suggesting that black hole spin has minimal influence on the accretion dynamics. In addition, we conduct post-processing using a two-temperature model to calculate the luminosities from synchrotron and bremsstrahlung radiation. We find that the total radiation luminosity is significantly higher than the luminosities from synchrotron and bremsstrahlung. This finding highlights the influence of radiation on the dynamics of the accretion flow. Our analysis shows that the electron temperature is significantly high in the jet region, regardless of spin. We further find that the temporal evolution of both radiative and synchrotron luminosities exhibits qualitatively similar behavior across all spin values. Finally, our results indicate that black hole spin has minimal impact on the spectral energy distribution (SED) in MAD state accretion flows.

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arXiv2025

Global resistive MHD accretion flows around spinning AGNs: impact of resistivity on MAD state

In this study, we investigate the effect of resistivity on the dynamics of global magnetohydrodynamic accretion flows (Res-MHD) around a spinning supermassive black hole. We perform a comparative study of 2D and 3D resistive models around black holes. We examine accretion flow dynamics considering globally uniform resistivity values, ranging from $\sim 0$ to 0.1. During the simulation time of $t \lesssim 1000~t_g$, we find that the mass accretion rate is comparable for both the 2D and 3D models. However, as the flow becomes increasingly turbulent, non-axisymmetric effects begin to dominate, resulting in significant differences in the mass accretion rates between the 3D and 2D. All the resistive models in a highly magnetized flow belong to the Magnetically Arrested Disk (MAD) state. We propose an efficient and physically motivated approach to examine the magnetic state by estimating the spatial average plasma beta parameter across the computational domain. We find that when the average plasma beta is close to or below unity $( \beta_{\text{ave}} \lesssim 1 )$, the accretion flow enters the MAD state. Additionally, we find that high-resistivity flow reduces magnetorotational instability (MRI) turbulence in the accretion flow, while the turbulence structures remain qualitatively similar in low-resistivity flows. Moreover, we observe indications of plasmoid formations in low-resistivity flow compared to high-resistivity flow. Furthermore, we do not find a clear relationship between the variability of the accretion rate, magnetic flux, and resistivity. Lastly, our findings suggest that low-resistivity models produce higher power jets than those with higher resistivity.

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arXiv2025

Numerical studies of (in)stabilities of shocks in perturbed advective flows around black holes

Using two-dimensional hydrodynamic simulations, we investigate the stability of shocked accretion flows around black holes under non-axisymmetric perturbations. By systematically exploring the parameter space of specific energy and angular momentum that permits shock formation in advective accretion flows, we demonstrate that quasi-periodic oscillations (QPOs) naturally emerge in perturbed systems. Our spectral analysis reveals characteristic QPO frequencies spanning 0.44-146.57 Hz, effectively bridging the observed low-frequency (LFQPOs) and high-frequency QPOs (HFQPOs) in black hole X-ray binaries. The quality factors of these oscillations range from 1.66 to 203.58, with multiple Lorentzian components indicating distinct oscillation modes. Through wavelet analysis and cross-validation with recent observations (e.g., Swift J1727.8-1613 and GX 339-4), we establish that shock instabilities driven by acoustic wave interactions between the non-axisymmetric perturbation and the shock location can quantitatively explain the temporal features observed in accreting black hole systems. Furthermore, we characterize the adiabatic index dependence of shock morphology, showing that increasing the adiabatic index from 4/3 to 1.4 changes shock positions outward while maintaining oscillation coherence.

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arXiv2024

Radiation RMHD accretion flows around spinning AGNs: a comparative study of MAD and SANE state

In our study, we examine a 2D radiation, relativistic, magnetohydrodynamics (Rad-RMHD) accretion flows around a spinning supermassive black hole. We begin by setting an initial equilibrium torus around the black hole, with an embedded initial magnetic field inside the torus. The strength of the initial magnetic field is determined by the plasma beta parameter, which is the ratio of the gas pressure to the magnetic pressure. In this paper, we perform a comparative study of the `magnetically arrested disc (MAD)' and `standard and normal evolution (SANE)' states. We observe that MAD state is possible for comparatively high initial magnetic field strength flow. Additionally, we also adopt a self-consistent two-temperature model to evaluate the luminosity and energy spectrum for our model. We observe that the total luminosity is mostly dominated by bremsstrahlung luminosity compared to the synchrotron luminosity due to the presence of highly dense torus. We also identify similar quasi-periodic oscillations (QPOs) for both MAD and SANE states based on power density spectrum analysis. Furthermore, our comparative study of the energy spectrum does not reveal any characteristic differences between MAD and SANE states. Lastly, we note that the MAD state is possible for both prograde and retrograde accretion flow.

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arXiv2023

Evolution of MHD Torus and Mass Outflow Around Spinning AGN

We perform axisymmetric, two-dimensional magnetohydrodynamic (MHD) simulations to investigate accretion flows around spinning AGN. To mimic the space-time geometry of spinning black holes, we consider effective Kerr potential, and the mass of the black holes is $10^8 M_{\odot}$. We initialize the accretion disc with a magnetized torus by adopting the toroidal component of the magnetic vector potential. The initial magnetic field strength is set by using the plasma beta parameter ($\beta_0$). We observe self-consistent turbulence generated by magneto rotational instability (MRI) in the disc. The MRI turbulence transports angular momentum in the disc, resulting in an angular momentum distribution that approaches a Keplerian distribution. We investigate the effect of the magnetic field on the dynamics of the torus and associated mass outflow from the disc around a maximally spinning black hole $(a_k = 0.99)$. For the purpose of our analysis, we investigate the magnetic state of our simulation model. The model $\beta_0 = 10$ indicates the behaviour similar to the "magnetically arrested disk (MAD)'' state, and all the other low magnetic model remains in the SANE state. We observe that mass outflow rates are significantly enhanced with the increased magnetic field in the disc. We find a positive correlation between the magnetic field and mass outflow rates. We also investigate the effect of black hole spin on the magnetized torus evolution. However, we have not found any significant effect of black hole spin on mass outflows in our model. Finally, we discuss the possible astrophysical applications of our simulation results.

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arXiv2023

Time delays between radio and X-ray and between narrow radio bands of Sgr A* flares in the shock oscillation model

We examine the time delay between radio and X-ray and between narrow radio frequency flares in Sagittarius A* (Sgr A*), from analyses of the synchrotron, bremsstrahlung and monochromatic luminosity curves. Using the results of 2D relativistic radiation magnetohydrodynamic (MHD) simulations based on the shock oscillation model, we find three types of time delay between the synchrotron and bremsstrahlung emissions: Type A with a time delay of 2 -- 3 h on the shock descending branch, Type B with no time delay and Type C with an inverse time delay of 0.5 -- 1 h on the shock ascending branch. The time delays in Types A and C are interpreted as a transit time of Alfv\'{e}n and acoustic waves between both emission dominant regions, respectively. The delay times between 22 and 43 GHz flares and between 8 and 10 GHz flares are $\sim$ 13 -- 26 min and 13 min, respectively, while the inverse delay also occurs dependently on the shock location branch. These time delays between the narrow radio bands are interpreted as the transit time of the acoustic wave between the frequency-dependent effective radii $R_{\tau_{\rm \nu=1}}$, at which the optical depth $\tau_{\rm \nu}$ at the accretion disc surface becomes $\sim$ unity. The shock oscillation model explains well the observed delay times of 0.5 -- 5 h between radio and X-ray, 20 -- 30 min between 22 and 43 GHz and $\sim$ 18 min between 8 and 10 GHz in Sgr A*.

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arXiv2021

Radiative Shocks around Super-Eddington Accreting Black Holes

We examine radiative standing shocks in advective accretion flows around stellar-mass black holes by 2D radiation hydrodynamic simulations, focusing on the super-Eddington accreting flow. Under a set of input flow parameters responsible for the standing shock, the shock location on the equator decreases toward the event horizon with an increasing accretion rate. The optically thin and hot gas in the narrow funnel region along the rotational axis changes gradually into a dense and optically thick state with the increasingly dense gas transported from the base of the radiative shock near the equator. As a result, the luminosity becomes as high as ~ $10^{40}$ erg $s^{-1}$, and the radiation shows a strongly anisotropic distribution around the rotational axis and then very low edge-on luminosity as ~ $10^{36}$ erg $s^{-1}$. The mass outflow rate from the outer boundary is high as ~ $10^{-5}$ and $10^{-4}$ $M_{\odot} yr^{-1}$ but most of the outflow is originated through the radial outer boundary and may be observed over a wide wind region. The models show approximately black body spectra with a temperature of $5 \times 10^{6} - 3 \times 10^{7}$ K at the vertical outer boundary surface. The radiative shock models with the super-Eddington luminosities show a possible model for the superaccretor SS 433 and Ultraluminous X-ray sources with stellar-mass black holes.

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arXiv2021

Effects of resistivity on standing shocks in low angular momentum flows around black holes

We study two dimensional low angular momentum flow around the black hole using the resistive magnetohydrodynamic module of PLUTO code. Simulations have been performed for the flows with parameters of specific angular momentum, specific energy, and magnetic field which may be expected for the flow around Sgr A*. For flows with lower resistivity $\eta=10^{-6}$ and $0.01$, the luminosity and the shock location on the equator vary quasi-periodically. The power density spectra of luminosity variation show the peak frequencies which correspond to the periods of $5 \times 10^5$, $1.4 \times 10^5$, and $5 \times 10^4$ seconds, respectively. These quasi-periodic oscillations (QPOs) occur due to the interaction between the outer oscillating standing shock and the inner weak shocks occurring at the innermost hot blob. While for cases with higher resistivity $\eta=0.1$ and 1.0, the high resistivity considerably suppresses the magnetic activity such as the MHD turbulence and the flows tend to be steady and symmetric to the equator. The steady standing shock is formed more outward compared with the hydrodynamical flow. The low angular momentum flow model with the above flow parameters and with low resistivity has a possibility for the explanation of the long-term flares with $\sim$ one per day and $\sim 5 - 10$ days of Sgr A* in the latest observations by Chandra, Swift, and XMM-Newton monitoring of Sgr A*.

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arXiv2019

A Possible Model for the Long-Term Flares of Sgr A*

We examine the effects of magnetic field on low angular momentum flows with standing shock around black holes in two dimensions. The magnetic field brings change in behavior and location of the shock which results in regularly or chaotically oscillating phenomena of the flow. Adopting fiducial parameters like specific angular momentum, specific energy and magnetic field strength for the flow around Sgr A*, we find that the shock moves back and forth in the range 60--170R_g, irregularly recurring with a time-scale of ~5 days with an accompanying more rapid small modulation with a period of 25 hrs without fading, where R_g is the Schwarzschild radius. The time variability associated with two different periods is attributed to the oscillating outer strong shock, together with another rapidly oscillating inner weak shock. As a consequence of the variable shock location, the luminosities vary roughly by more than a factor of 3. The time-dependent behaviors of the flow are well compatible with luminous flares with a frequency of ~one per day and bright flares occurring every ~5 -- 10 days in the latest observations by Chandra, Swift and XMM-Newton monitoring of Sgr A*.

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arXiv2015

Unstable mass-outflows in geometrically thick accretion flows around black holes

Accretion flows around black holes generally result in mass-outflows that exhibit irregular behavior quite often. Using 2D time-dependent hydrodynamical calculations, we show that the mass-outflow is unstable in the cases of thick accretion flows such as the low angular momentum accretion flow and the advection-dominated accretion flow. For the low angular momentum flow, the inward accreting matter on the equatorial plane interacts with the outflowing gas along the rotational axis and the centrifugally supported oblique shock is formed at the interface of both the flows, when the viscosity parameter $α$ is as small as $α\le 10^{-3}$. The hot and rarefied blobs, which result in the eruptive mass-outflow, are generated in the inner shocked region and grow up toward the outer boundary. The advection-dominated accretion flow attains finally in the form of a torus disc with the inner edge of the disc at $3R_{\rm g} \le r \le 6R_{\rm g}$ and the center at $6R_ {\rm g} \le r \le 10R_{\rm g}$, and a series of hot blobs is intermittently formed near the inner edge of the torus and grows up along the outer surface of the torus. As a result, the luminosity and the mass-outflow rate are modulated irregularly where the luminosity is enhanced by 10-40% and the mass-outflow rate is increaed by a factor of few up to ten. We interpret the unstable nature of the outflow to be due to the Kelvin-Helmholtz instability, examining the Richardson number for the Kelvin-Helmholtz criterion in the inner region of the flow. We propose that the flare phenomena of Sgr A* may be induced by the unstable mass-outflow as is found in this work.

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arXiv2014

Low angular momentum flow model II for Sgr A*

We examine 1D two-temperature accretion flows around a supermassive black hole, adopting the specific angular momentum λ, the total specific energy εand the input accretion rate \dot M_{input} = 4.0x10^{-6} solar mass/yr estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The two-temperature flow is almost adiabatic even if we take account of the heating of electrons by ions, the bremsstrahlung cooling and the synchrotron cooling, as long as the ratio βof the magnetic energy density to the thermal energy density is taken to be as β< 1. The different temperatures of ions and electrons are caused by the different adiabatic indices of ions and electrons which depend on their temperature states under the relativistic regime. The total luminosity increases with increasing βand results in - 10^{35} - 10^{36} erg/s for β=10^{-3} - 1. Furthermore, from 2D time-dependent hydrodynamical calculations of the above flow, we find that the irregularly oscillating shocks are formed in the inner region and that the luminosity and the mass-outflow rate vary by a factor of 2 -- 3 and 1.5 -- 4, respectively. The time variability may be relevant to the flare activity of Sgr A*.

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arXiv2012

Low angular momentum flow model for Sgr A*

We examine the low angular momentum flow model for Sgr A* using two-dimensional hydrodynamical calculations based on the parameters of the specific angular momentum and total energy estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The accretion flow with the plausible parameters is non-stationary and an irregularly oscillating shock is formed in the inner region of a few tens to a hundred and sixty Schwarzschild radii. Due to the oscillating shock, the luminosity and the mass-outflow rate are modulated by several per cent to a factor of 5 and a factor of 2-7, respectively, on time-scales of an hour to ten days. The flows are highly advected and the radiative efficiency of the accreting matter into radiation is very low, 10^{-5}--$10^{-3}, and the input accretion rate of 4.0* 10^{-6} solar mass/yr results in the observed luminosities -- 10^{36} erg/s of Sgr A* if a two-temperature model and the synchrotron emission are taken into account. The mass-outflow rate of the gas originating in the post-shock region increases with the increasing input specific angular momentum and ranges from a few to 99 per cent of the input accreting matter, depending on the input angular momentum. The oscillating shock is necessarily triggered if the specific angular momentum and the specific energy belong to or are located just nearby in the range of parameters responsible for a stationary shock in rotating inviscid and adiabatic accretion flow. The time variability may be relevant to the flare activity of Sgr A*.

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arXiv2009

The Jets and Disc of SS 433 at Super-Eddington Luminosities

We examine the jets and the disc of SS 433 at super-Eddington luminosities with 600 times Eddington critical accretion rate by time-dependent two-dimensional radiation hydrodynamical calculations, assuming alpha-model for the viscosity. One-dimensional supercritical accretion disc models with mass loss or advection are used as the initial configurations of the disc. As a result, from the initial advective disc models with alpha =0.001 and 0.1, we obtain the total luminosities 2.5x10^{40} and 2.0x10^{40} erg/s. The total mass-outflow rates are 4x10^{-5} and 10^{-4} solar-mass/yr and the rates of the relativistic axial outflows in a small half opening angle of 1 degree are about 10^{-6} solar-mass/yr: the values generally consistent with the corresponding observed rates of the wind and the jets, respectively. From the initial models with mass loss but without advection, we obtain the total mass-outflow and axial outflow rates smaller than or comparable to the observed rates of the wind and the jets respectively, depending on alpha. In the advective disc model with alpha=0.1, the initially radiation-pressure dominant, optically thick disc evolves to the gas-pressure dominated, optically thin state in the inner region of the disc, and the inner disc is unstable. Consequently, we find remarkable modulations of the disc luminosity and the accretion rate through the inner edge. These modulations manifest themselves as the recurrent hot blobs with high temperatures and low densities at the disc plane, which develop outward and upward and produce the QPOs-like variability of the total luminosity with an amplitude of a factor of 2 and quasi-periods of 10 -- 25 s. This may explain the massive jet ejection and the QPOs phenomena observed in SS 433.

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arXiv2002

Super-Eddington Black-Hole Models for SS 433

We examine highly super-Eddington black-hole models for SS 433, based on two-dimensional hydrodynamical calculations coupled with radiation transport. The super-Eddington accretion flow with a small viscosity parameter, $α= 10^{-3}$, results in a geometrically and optically thick disk with a large opening angle of $\sim 60^{\circ}$ to the equatorial plane and a very rarefied, hot, and optically thin high-velocity jets region around the disk. The thick accretion flow consists of two different zones: an inner advection-dominated zone and an outer convection-dominated zone. The high-velocity region around the disk is divided into two characteristic regions, a very rarefied funnel region along the rotational axis and a moderately rarefied high-velocity region outside of the disk. The temperatures of $\sim 10^7$ K and the densities of $\sim 10^{-7}$ g cm$^{-3}$ in the upper disk vary sharply to $\sim 10^8$ K and $10^{-8}$ g cm$^{-3}$, respectively, across the disk boundary between the disk and the high-velocity region. The X-ray emission of iron lines would be generated only in a confined region between the funnel wall and the photospheric disk boundary, where flows are accelerated to relativistic velocities of $\sim$ 0.2 $c$ due to the dominant radiation-pressure force. The results are discussed regarding the collimation angle of the jets, the large mass-outflow rate obserevd in SS 433, and the ADAFs and the CDAFs models.

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