212 research outputs found
A Novel Jet Model: Magnetically Collimated, Radiation-Pressure Driven Jet
Relativistic jets from compact objects are ubiquitous phenomena in the
Unvierse, but their driving mechanism has been an enigmatic issue over many
decades. Two basic models have been extensively discussed: magnetohydrodynamic
(MHD) jets and radiation-hydrodynamic (RHD) jets. Currently, the former is more
widely accepted, since magnetic field is expected to provide both the
acceleration and collimation mechanisms, whereas radiation field cannot
collimate outflow. Here, we propose a new type of jets,
radiation-magnetohydrodynamic (RMHD) jets, based on our global RMHD simulation
of luminous accretion flow onto a black hole shining above the Eddington
luminosity. The RMHD jet can be accelerated up to the relativistic speed by the
radiation-pressure force and is collimated by the Lorentz force of a magnetic
tower, inflated magnetic structure made by toroidal magnetic field lines
accumulated around the black hole, though radiation energy greatly dominates
over magnetic energy. This magnetic tower is collimated by a geometrically
thick accretion flow supported by radiation-pressure force. This type of jet
may explain relativistic jets from Galactic microquasars, appearing at high
luminosities.Comment: 5 pages, 2 figures, accepted for publication in PAS
Spectral energy distribution of super-Eddington flows
Spectral properties of super-Eddington accretion flows are investigated by
means of a parallel line-of-sight calculation. The subjacent model, taken from
two-dimensional radiation hydrodynamic simulations by Ohsuga et al. (2005),
consists of a disc accretion region and an extended atmosphere with high
velocity outflows. The non-gray radiative transfer equation is solved,
including relativistic effects, by applying the FLD approximation.
The calculated spectrum is composed of a thermal, blackbody-like emission
from the disc which depends sensitively on the inclination angle, and of high
energy X-ray and gamma-ray emission from the atmosphere. We find mild beaming
effects in the thermal radiation for small inclination angles. If we compare
the face-on case with the edge-on case, the average photon energy is larger by
a factor of ~1.7 due mainly to Doppler boosting, while the photon number
density is larger by a factor of ~3.7 due mainly to anisotropic matter
distribution around the central black hole. This gives an explanation for the
observed X-ray temperatures of ULXs which are too high to be explained in the
framework of intermediate-mass black holes.
While the main features of the thermal spectral component are consistent with
more detailed calculations of slim accretion discs, the atmosphere induces
major changes in the high-energy part, which cannot be reproduced by existing
models. In order to interpret observational data properly, simple approaches
like the Eddington-Barbier approximation cannot be applied.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Geometrical Effect of Supercritical Accretion Flows: Observational Implications of Galactic Black-Hole Candidates and Ultraluminous X-ray Sources
We investigate the dependence of the viewing angle in supercritical accretion
flows and discuss the observational implications of galactic black-hole
candidates and ultraluminous X-ray sources. When the mass accretion rate
exceeds the critical rate, then the shape of the disk is geometrically thick
due to the enhanced radiation pressure. The model spectra of supercritical
accretion flows strongly depend on the inclination angle. Because the outer
disk blocks the emission from the disk inner region for high inclination angle.
We also find that the spectral properties of low-inclination angle and low
accretion-rate disks are very similar to those of high-inclination and high
accretion rate disks. That is, if an object has a high inclination and high
accretion rate, such a system suffers from self-occultation and the spectrum
will be extremely soft. Therefore, we cannot discriminate these differences
from spectrum shapes only. Conversely, if we use the self-occultation
properties, we could constrain the inclination angle of the system. We suggest
that some observed high temperature ultraluminous X-ray sources have near
face-on geometry, i < 40, and Galactic black hole candidate, XTE J1550-564,
possesses relatively high-inclination angles, i > 60.Comment: 13 pages, 6 figures, accepted for publication in PAS
Revisiting vertical structure of neutrino-dominated accretion disks: Bernoulli parameter, neutrino trapping and other distributions
We revisit the vertical structure of neutrino dominated accretion flows
(NDAFs) in spherical coordinates with a new boundary condition based on the
mechanical equilibrium. The solutions show that NDAF is significantly thick.
The Bernoulli parameter and neutrino trapping are determined by the mass
accretion rate and the viscosity parameter. According to the distribution of
the Bernoulli parameter, the possible outflow may appear in the outer region of
the disk. The neutrino trapping can essentially affect the neutrino radiation
luminosity. The vertical structure of NDAF is like a "sandwich", and the
multilayer accretion may account for the flares in gamma-ray bursts.Comment: 7 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Does the Slim-Disk Model Correctly Consider Photon-Trapping Effects?
We investigate the photon-trapping effects in the super-critical black hole
accretion flows by solving radiation transfer as well as the energy equations
of radiation and gas. It is found that the slim-disk model generally
overestimates the luminosity of the disk at around the Eddington luminosity
(L_E) and is not accurate in describing the effective temperature profile,
since it neglects time delay between energy generation at deeper inside the
disk and energy release at the surface. Especially, the photon-trapping effects
are appreciable even below L ~ L_E, while they appear above ~ 3L_E according to
the slim disk. Through the photon-trapping effects, the luminosity is reduced
and the effective temperature profile becomes flatter than r^{-3/4} as in the
standard disk. In the case that the viscous heating is effective only around
the equatorial plane, the luminosity is kept around the Eddington luminosity
even at very large mass accretion rate, Mdot>>L_E/c^2. The effective
temperature profile is almost flat, and the maximum temperature decreases in
accordance with rise in the mass accretion rate. Thus, the most luminous radius
shifts to the outer region when Mdot/(L_E/c^2) >> 10^2. In the case that the
energy is dissipated equally at any heights, the resultant luminosity is
somewhat larger than in the former case, but the energy-conversion efficiency
still decreases with increase of the mass accretion rate, as well. The most
luminous radius stays around the inner edge of the disk in the latter case.
Hence, the effective temperature profile is sensitive to the vertical
distribution of energy production rates, so is the spectral shape. Future
observations of high L/L_E objects will be able to test our model.Comment: 10 pages, 7 figures, accepted for publication in Ap
Why Is Supercritical Disk Accretion Feasible?
Although the occurrence of steady supercritical disk accretion onto a black
hole has been speculated about since the 1970s, it has not been accurately
verified so far. For the first time, we previously demonstrated it through
two-dimensional, long-term radiation-hydrodynamic simulations. To clarify why
this accretion is possible, we quantitatively investigate the dynamics of a
simulated supercritical accretion flow with a mass accretion rate of ~10^2
L_E/c^2 (with L_E and c being, respectively, the Eddington luminosity and the
speed of light). We confirm two important mechanisms underlying supercritical
disk accretion flow, as previously claimed, one of which is the radiation
anisotropy arising from the anisotropic density distribution of very optically
thick material. We qualitatively show that despite a very large radiation
energy density, E_0>10^2L_E/(4 pi r^2 c) (with r being the distance from the
black hole), the radiative flux F_0 cE_0/tau could be small due to a large
optical depth, typically tau 10^3, in the disk. Another mechanism is photon
trapping, quantified by vE_0, where v is the flow velocity. With a large |v|
and E_0, this term significantly reduces the radiative flux and even makes it
negative (inward) at r<70r_S, where r_S is the Schwarzschild radius. Due to the
combination of these effects, the radiative force in the direction along the
disk plane is largely attenuated so that the gravitational force barely exceeds
the sum of the radiative force and the centrifugal force. As a result, matter
can slowly fall onto the central black hole mainly along the disk plane with
velocity much less than the free-fall velocity, even though the disk luminosity
exceeds the Eddington luminosity. Along the disk rotation axis, in contrast,
the strong radiative force drives strong gas outflows.Comment: 8 pages, 7 figures, accepted for publication in Ap
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