106 research outputs found

### Relativistic Radiative Flow in a Luminous Disk

Radiatively driven transfer flow perpendicular to a luminous disk was
examined under a fully special relativistic treatment, taking into account
radiation transfer. The flow was assumed to be vertical, and the gravity, the
gas pressure, and the viscous heating were ignored. In order to construct the
boundary condition at the flow top, the magic speed above the flat source was
re-examined, and it was found that the magic speed above a moving source can
exceed that above a static source ($\sim 0.45~c$). Then, the radiatively driven
flow in a luminous disk was numerically solved, from the flow base (disk
``inside''), where the flow speed is zero, to the flow top (disk ``surface''),
where the optical depth is zero. For a given optical depth and appropriate
initial conditions at the flow base, where the flow starts, a loaded mass in
the flow was obtained as an eigenvalue of the boundary condition at the flow
top. Furthermore, a loaded mass and the flow final speed at the flow top were
obtained as a function of the radiation pressure at the flow base; the flow
final speed increases as the loaded mass decreases. Moreover, the flow velocity
and radiation fields along the flow were obtained as a function of the optical
depth. Within the present treatment, the flow three velocity $v$ is restricted
to be within the range of $v < c/\sqrt{3}$, which is the relativistic sound
speed, due to the relativistic effect.Comment: 8 pages, 5 figure

### Relativistic Radiation Hydrodynamical Accretion Disk Winds

Accretion disk winds browing off perpendicular to a luminous disk are
examined in the framework of fully special relativistic radiation
hydrodynamics. The wind is assumed to be steady, vertical, and isothermal. %and
the gravitational fields is approximated by a pseudo-Newtonian potential. Using
a velocity-dependent variable Eddington factor, we can solve the rigorous
equations of relativistic radiative hydrodynamics, and can obtain radiatively
driven winds accelerated up to the {\it relativistic} speed. For less luminous
cases, disk winds are transonic types passing through saddle type critical
points, and the final speed of winds increases as the disk flux and/or the
isothermal sound speed increase. For luminous cases, on the other hand, disk
winds are always supersonic, since critical points disappear due to the
characteristic nature of the disk gravitational fields. The boundary between
the transonic and supersonic types is located at around $\hat{F}_{\rm c} \sim
0.1 (\epsilon+p)/(\rho c^2)/\gamma_{\rm c}$, where $\hat{F}_{\rm c}$ is the
radiative flux at the critical point normalized by the local Eddington
luminosity, $(\epsilon+p)/(\rho c^2)$ is the enthalpy of the gas divided by the
rest mass energy, and $\gamma_{\rm c}$ is the Lorentz factor of the wind
velocity at the critical point. In the transonic winds, the final speed becomes
0.4--0.8$c$ for typical parameters, while it can reach $\sim c$ in the
supersonic winds.Comment: 6 pages, 5 figures; PASJ 59 (2007) in pres

### Radiative Transfer and Limb Darkening of Accretion Disks

Transfer equation in a geometrically thin accretion disk is reexamined under
the plane-parallel approximation with finite optical depth. Emergent intensity
is analytically obtained in the cases with or without internal heating. For
large or infinite optical depth, the emergent intensity exhibits a usual
limb-darkening effect, where the intensity linearly changes as a function of
the direction cosine. For small optical depth, on the other hand, the
angle-dependence of the emergent intensity drastically changes. In the case
without heating but with uniform incident radiation at the disk equator, the
emergent intensity becomes isotropic for small optical depth. In the case with
uniform internal heating, the limb brightening takes place for small optical
depth. We also emphasize and discuss the limb-darkening effect in an accretion
disk for several cases.Comment: 7 pages, 4 figure

### Relativistic Radiative Flow in a Luminous Disk II

Radiatively-driven transfer flow perpendicular to a luminous disk is examined
in the relativistic regime of $(v/c)^2$, taking into account the gravity of the
central object. The flow is assumed to be vertical, and the gas pressure as
well as the magnetic field are ignored. Using a velocity-dependent variable
Eddington factor, we can solve the rigorous equations of the relativistic
radiative flow accelerated up to the {\it relativistic} speed. For sufficiently
luminous cases, the flow resembles the case without gravity. For less-luminous
or small initial radius cases, however, the flow velocity decreases due to
gravity. Application to a supercritical accretion disk with mass loss is
briefly discussed.Comment: 7 pages, 5 figure

### Hoyle-Lyttleton Accretion onto Accretion Disks

We investigate Hoyle-Lyttleton accretion for the case where the central
source is a luminous accretion disk. %In classical Hoyle-Lyttleton accretion
onto a ``spherical'' source, accretion takes place in an axially symmetric
manner around a so-called accretion axis. The accretion rate of the classical
Hoyle-Lyttleton accretion onto a non-luminous object and $\Gamma$ the
luminosity of the central object normalized by the Eddington luminosity. %If
the central object is a compact star with a luminous accretion disk, the
radiation field becomes ``non-spherical''. %Although the gravitional field
remains spherical. In such a case the axial symmetry around the accretion axis
breaks down; the accretion radius $R_{acc}$ generally depends on an inclination
angle $i$ between the accretion axis and the symmetry axis of the disk and the
azimuthal angle $\phi$ around the accretion axis. %That is, the cross section
of accretion changes its shape. Hence, the accretion rate $\dot{M}$, which is
obtained by integrating $R_{acc}$ around $\phi$, depends on $i$. % as well as
$M$, $\Gamma$, and $v_\infty$. %In the case of an edge-on accretion
($i=90^{\circ}$), The accretion rate is larger than that of the spherical case
and approximately expressed as $\dot{M} \sim \dot{M}_{HL} (1-\Gamma)$ for
$\Gamma \leq 0.65$ and $\dot{M} \sim \dot{M}_{HL} (2-\Gamma)^2/5$ for $\Gamma
\geq 0.65$. %Once the accretion disk forms and the anisotropic radiation fields
are produced around the central object,the accretion plane will be maintained
automatically (the direction of jets associated with the disk is also
maintained). %Thus, the anisotropic radiation field of accretion disks
drastically changes the accretion nature, that gives a clue to the formation of
accretion disks around an isolated black hole.Comment: 5 figure

### Optical Light Curves of Luminous Eclipsing Black Hole X-ray Binaries

We examine optical V-band light curves in luminous eclipsing black hole X-ray
binaries, using a supercritical accretion/outflow model that is more realistic
than the formerly used ones. In order to compute the theoretical light curve in
the binary system, we do not only apply the global analytic solution of the
disk, but also include the effect of the optically thick outflow. We found that
the depth of eclipse of the companion star by the disk changes dramatically
when including the effect of the outflow. Due to the effect of outflow, we can
reproduce the optical light curve for typical binary parameters in SS433. Our
model with outflow velocity v~3000 km/s can fit whole shape of the averaged
V-band light curve in SS433, but we found a possible parameter range consistent
with observations, such as \dot{M}~5000-10000 L_E/c^2 (with L_E being the
Eddington luminosity and $c$ being the speed of light) and T_C~10000K-14000 K
for the accretion rate and donor star temperature, respectively. Furthermore,
we briefly discuss observational implications for ultraluminous X-ray sources.Comment: 8 pages, 9 figures, accepted for publication in PAS

### Radiative Flow in a Luminous Disk

Radiatively-driven flow in a luminous disk is examined in the subrelativistic
regime of $(v/c)^1$, taking account of radiation transfer. The flow is assumed
to be vertical, and the gravity and gas pressure are ignored. When internal
heating is dropped, for a given optical depth and radiation pressure at the
flow base (disk ``inside''), where the flow speed is zero, the flow is
analytically solved under the appropriate boundary condition at the flow top
(disk ``surface''), where the optical depth is zero. The loaded mass and
terminal speed of the flow are both determined by the initial conditions; the
mass-loss rate increases as the initial radiation pressure increases, while the
flow terminal speed increases as the initial radiation pressure and the loaded
mass decrease. In particular, when heating is ignored, the radiative flux $F$
is constant, and the radiation pressure $P_0$ at the flow base with optical
depth $\tau_0$ is bound in the range of $2/3 < cP_0/F < 2/3 + \tau_0$. In this
case, in the limit of $cP_0/F = 2/3 + \tau_0$, the loaded mass diverges and the
flow terminal speed becomes zero, while, in the limit of $cP_0/F = 2/3$, the
loaded mass becomes zero and the terminal speed approaches $(3/8)c$, which is
the terminal speed above the luminous flat disk under an approximation of the
order of $(v/c)^1$. We also examine the case where heating exists, and find
that the flow properties are qualitatively similar to the case without heating.Comment: 7 pages, 4 figure

### Effect of Radiation Drag on Hoyle-Lyttleton Accretion

Hoyle-Lyttleton type accretion is investigated, by taking account of not only
the effect of radiation pressure but the effect of radiation drag. We calculate
the trajectories of particles for three cases: only the effect of gravity is
considered (case A); the effect of radiation pressure is taken into account
(case B); the effect of radiation drag as well as radiation pressure is taken
into account (case C). The accretion radii for former two cases are
$2GM/v_{\infty}^2$ for case A and $2GM(1-\Gamma)/v_{\infty}^2$ for case B,
where M is the mass of the accreted object, $v_{\infty}$ the relative velocity,
and Gamma the normalized luminosity of the accreted object. We found that the
accretion radius for case C is in between those of cases A and B under the
present approximation; i.e., the accretion radius decreases due to radiation
pressure while it increases due to radiation drag. In addition, the accretion
radius for case C becomes larger as the incident velocity becomes fast. The
effect of radiation drag becomes more and more important when the velocity of
the incident particle is comparable to the light speed.Comment: 11 pages, LaTeX with 6 eps figures, accepted by Publications of the
Astronomical Society of Japa

### Radiative Transfer in Accretion-Disk Winds

Radiative transfer equation in an accretion disk wind is examined
analytically and numerically under the plane-parallel approximation in the
subrelativistic regime of $(v/c)^1$, where $v$ is the wind vertical velocity.
Emergent intensity is analytically obtained for the case of a large optical
depth, where the flow speed and the source function are almost constant. The
usual limb-darkening effect, which depends on the direction cosine at the
zero-optical depth surface, does not appear, since the source function is
constant. Because of the vertical motion of winds, however, the emergent
intensity exhibits the {\it velocity-dependent} limb-darkening effect, which
comes from the Doppler and aberration effects. Radiative moments and emergent
intensity are also numerically obtained. When the flow speed is small ($v \leq
0.1c$), the radiative structure resembles to that of the static atmosphere,
where the source function is proportional to the optical depth, and the usual
limb-darkening effect exists. When the flow speed becomes large, on the other
hand, the flow speed attains the constant terminal one, and the
velocity-dependent limb-darkening effect appears. We thus carefully treat and
estimate the wind luminosity and limb-darkening effect, when we observe an
accretion disk wind.Comment: 8 pages, 5 figure

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