ABSTRACT Using three-dimensional general relativistic radiation-magnetohydrodynamics simulations of accretion flows around stellar mass black holes, we report that the relatively cold disk ( ) is ...truncated near the black hole. Hot and less dense regions, of which the gas temperature is and more than 10 times higher than the radiation temperature (overheated regions), appear within the truncation radius. The overheated regions also appear above as well as below the disk, sandwiching the cold disk, leading to the effective Compton upscattering. The truncation radius is for , where are the gravitational radius, mass accretion rate, Eddington luminosity, and light speed, respectively. Our results are consistent with observations of a very high state, whereby the truncated disk is thought to be embedded in the hot rarefied regions. The truncation radius shifts inward to with increasing mass accretion rate , which is very close to an innermost stable circular orbit. This model corresponds to the slim disk state observed in ultraluminous X-ray sources. Although the overheated regions shrink if the Compton cooling effectively reduces the gas temperature, the sandwich structure does not disappear at the range of . Our simulations also reveal that the gas temperature in the overheated regions depends on black hole spin, which would be due to efficient energy transport from black hole to disks through the Poynting flux, resulting in gas heating.
By performing 2.5-dimensional general relativistic radiation magnetohydrodynamic simulations, we demonstrate supercritical accretion onto a non-rotating, magnetized neutron star, where the magnetic ...field strength of dipole fields is 1010 G on the star surface. We found the supercritical accretion flow consists of two parts: the accretion columns and the truncated accretion disk. The supercritical accretion disk, which appears far from the neutron star, is truncated at around 3 R* (R* = 106 cm is the neutron star radius), where the magnetic pressure via the dipole magnetic fields balances with the radiation pressure of the disks. The angular momentum of the disk around the truncation radius is effectively transported inward through magnetic torque by dipole fields, inducing the spin up of a neutron star. The evaluated spin-up rate, ∼−10−11 s s−1, is consistent with the recent observations of the ultraluminous X-ray pulsars. Within the truncation radius, the gas falls onto a neutron star along the dipole fields, which results in a formation of accretion columns onto the northern and southern hemispheres. The net accretion rate and the luminosity of the column are 66 LEdd/c2 and 10 LEdd, where LEdd is the Eddington luminosity and c is the light speed. Our simulations support a hypothesis whereby the ultraluminous X-ray pulsars are powered by the supercritical accretion onto the magnetized neutron stars.
Abstract
We present a general relativistic radiative transfer code
RAIKOU
(来光) for multiwavlength studies of spectra and images including the black hole shadows around Kerr black holes. Important ...radiative processes in hot plasmas around black holes, i.e., (cyclo-)synchrotron, bremsstrahlung emission/absorption, and Compton/inverse-Compton scattering, are incorporated. The Maxwell–Jüttner and single/broken power-law electron distribution functions are implemented to calculate the radiative transfer via both thermal and nonthermal electrons. Two calculation algorithms are implemented for studies of the images and broadband spectra. An observer-to-emitter ray-tracing algorithm, which inversely solves the radiative transfer equation from the observer screen to emitting plasmas, is suitable for an efficient calculations of the images, e.g., the black hole shadows observed by the Event Horizon Telescope, and spectra without Compton effects. On the other hand, an emitter-to-observer Monte Carlo algorithm, by which photons are transported with a Monte Carlo method including the effects of Compton/inverse-Compton scatterings, enables us to compute multiwavelength spectra, with their energy bands broadly ranging from radio to very high energy gamma-ray. The X-ray black hole shadows, which are formed via synchrotron emission and inverse-Compton scattering processes and will be observed in the future X-ray interferometry missions, can be also computed with this algorithm. The code is generally applicable to accretion flows around Kerr black holes with relativistic jets and winds/coronae with various mass accretion rates (i.e., radiatively inefficient accretion flows, super-Eddington accretion flows, and others). We demonstrate an application of the code to a radiatively inefficient accretion flow onto a supermassive black hole.
We develop a general relativistic radiation magnetohydrodynamics (GR-RMHD) code inazuma in which the time-dependent radiation transfer equation (frequency-integrated Boltzmann equation) is solved in ...curved spacetime. The Eddington tensor is derived from the specific intensity, and we solve the zeroth and first moment equations in order to update the radiation fields. Therefore, our code can solve the radiation field around relativistic compact objects more appropriately than an approximation method like the M1 closure scheme. The numerical scheme of magnetohydrodynamics is the same as that of our previous code. In some test calculations for propagating radiation and radiation hydrodynamics in flat spacetime, our code shows similar results to our previous work. Radiation propagation in curved spacetime is also properly solved for. We also show the radiation transport from the super-Eddington accretion disk around the black hole. The disk structure, such as the density, velocity, and temperature, is fixed by the model obtained using the GR-RMHD simulation with the M1 method. We found that the difference between our scheme and the M1 method appears in the optically thin outflow region around the rotation axis while the radiation field is almost the same in the optically thick disk region.
Abstract
By performing two-dimensional axisymmetric general relativistic radiation magnetohydrodynamics simulations with spin parameter
a
* varying from −0.9 to 0.9, we investigate the dependence on ...the black hole spin of the energy flow from a supercritical accretion disk around a stellar mass black hole. It is found that optically and geometrically thick disks form near the equatorial plane, and a part of the disk matter is launched from the disk surface in all models. The gas ejection is mainly driven by the radiative force, but magnetic force cannot be neglected when ∣
a
*∣ is large. The energy outflow efficiency (total luminosity normalized by
M
̇
in
c
2
;
M
̇
in
and
c
are the mass-accretion rate at the event horizon and the light speed) is higher for rotating black holes than for nonrotating black holes. This is 0.7% for
a
* = −0.7, 0.3% for
a
* = 0, and 5% for
a
* = 0.7 for
M
̇
in
∼
100
L
Edd
/
c
2
(
L
Edd
is the Eddington luminosity). Furthermore, although the energy is mainly released by radiation when
a
* ∼ 0, the Poynting power increases with ∣
a
*∣ and exceeds the radiative luminosity for models with
a
* ≥ 0.5 and
a
* ≤ −0.7. The faster the black hole rotates, the higher the power ratio of the kinetic luminosity to the isotropic luminosity tends to be. This implies that objects with a high (low) power ratio may have rapidly (slowly) rotating black holes. Among ultraluminous X-ray sources, IC342 X-1, is a candidate with a rapidly rotating black hole.
Abstract
We perform general relativistic radiation magnetohydrodynamics simulations of super-Eddington accretion flows around a neutron star with a dipole magnetic field for modeling the Galactic ...ultraluminous X-ray source exhibiting X-ray pulsations, Swift J0243.6+6124. Our simulations show the accretion columns near the magnetic poles, the accretion disk outside the magnetosphere, and the outflows from the disk. It is revealed that the effectively optically thick outflows, consistent with the observed thermal emission at ∼10
7
K, are generated if the mass accretion rate is much higher than the Eddington rate
M
̇
Edd
and the magnetospheric radius is smaller than the spherization radius. In order to explain the blackbody radius (∼100–500 km) without contradicting the reported spin period (9.8 s) and spin-up rate (
P
̇
=
−
2.22
×
10
−
8
s
s
−
1
), a mass accretion rate of
(
200
–
1200
)
M
̇
Edd
is required. Since the thermal emission was detected in two observations with
P
̇
of −2.22 × 10
−8
s s
−1
and −1.75 × 10
−8
s s
−1
but not in another with
P
̇
=
−
6.8
×
10
−
9
s
s
−
1
, the surface magnetic field strength of the neutron star in Swift J0243.6+6124 is estimated to be between 3 × 10
11
G and 4 × 10
12
G. From this restricted range of magnetic field strength, the accretion rate would be
(
200
–
500
)
M
̇
Edd
when the thermal emission appears and
(
60
–
100
)
M
̇
Edd
when it is not detected. Our results support the hypothesis that the super-Eddington phase in the 2017–2018 giant outburst of Swift J0243.6+6124 is powered by highly super-Eddington accretion flows onto a magnetized neutron star.
ABSTRACT We develop a numerical scheme for solving the equations of fully special relativistic, radiation magnetohydrodynamics (MHDs), in which the frequency-integrated, time-dependent radiation ...transfer equation is solved to calculate the specific intensity. The radiation energy density, the radiation flux, and the radiation stress tensor are obtained by the angular quadrature of the intensity. In the present method, conservation of total mass, momentum, and energy of the radiation magnetofluids is guaranteed. We treat not only the isotropic scattering but also the Thomson scattering. The numerical method of MHDs is the same as that of our previous work. The advection terms are explicitly solved, and the source terms, which describe the gas-radiation interaction, are implicitly integrated. Our code is suitable for massive parallel computing. We present that our code shows reasonable results in some numerical tests for propagating radiation and radiation hydrodynamics. Particularly, the correct solution is given even in the optically very thin or moderately thin regimes, and the special relativistic effects are nicely reproduced.
To understand why supercritical accretion is feasible onto a neutron star (NS), we carefully examine the accretion flow dynamics by 2.5-dimensional general relativistic radiation magnetohydrodynamic ...(RMHD) simulations, comparing the cases of accretion onto a non-magnetized NS and that onto a black hole (BH). Supercritical BH accretion is relatively easy, since BHs can swallow excess radiation energy, so that radiation flux can be inward in its vicinity. This mechanism can never work for an NS, which has a solid surface. In fact, we find that the radiation force is always outward. Instead, we found significant reduction in the mass accretion rate due to strong radiation-pressure-driven outflow. The radiation flux Frad is self-regulated such that the radiation force balances with the sum of gravity and centrifugal forces. Even when the radiation energy density greatly exceeds that expected from the Eddington luminosity E rad F rad τ c > 10 2 L Edd ( 4 π r 2 c ) , the radiation flux is always kept below a certain value, which makes it possible not to blow all the gas away from the disk. These effects make supercritical accretion feasible. We also find that a settling region, where accretion is significantly decelerated by a radiation cushion, is formed around the NS surface. In the settling region, the radiation temperature and mass density roughly follow T rad ∝ r − 1 and ∝ r − 3 , respectively. No settling region appears around the BH, so matter can be directly swallowed by the BH with supersonic speed.
We construct a relativistic resistive magneto-hydrodynamic (RRMHD) numerical simulation code for high-energy heavy-ion collisions as a first designed code in the Milne coordinates. We split the ...system of differential equations into two parts, a non-stiff and a stiff part. For the non-stiff part, we evaluate the numerical flux using HLL approximated Riemann solver and execute the time integration by the second-order of Runge–Kutta algorithm. For the stiff part, which appears in Ampere’s law, we integrate the equations using semi-analytic solutions of the electric field. We employ the generalized Lagrange multiplier method to ensure the divergence-free constraint for the magnetic field and Gauss’s law. We confirm that our code reproduces well the results of standard RRMHD tests in the Cartesian coordinates. In the Milne coordinates, the code with high conductivity is validated against relativistic ideal MHD tests. We also verify the semi-analytic solutions of the accelerating longitudinal expansion of relativistic resistive magneto-hydrodynamics in high-energy heavy-ion collisions in comparison with our numerical result. Our numerical code reproduces these solutions.
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