155 research outputs found
Structure of ADAFs in a general large-Scale B-field: The role of wind and thermal conduction
We have explored the structure of hot flow bathed in a general large-scale
magnetic field. The importance of outflow and thermal conduction on the
self-similar structure of a hot accretion flows has been investigated. We
consider the additional magnetic parameters , where are the
Alfvn sound speeds in three direction of cylindrical coordinate. In
comparison to the accretion disk without winds, our results show that the
radial and rotational velocities of the disk become faster however it become
cooler because of the angular momentum and energy flux which are taking away by
the winds. but thermal conduction opposes the effect of winds not only decrease
the rotational velocity but also increase the radial velocity as well as the
sound speed of the disk. In addition we study the effect of global magnetic
field on the structure of the disk. Our numerical results show that all
components of magnetic field can be important and they have a considerable
effect on velocities and vertical structure of the disk.Comment: accepted for publication in Research in Astronomy and Astrophysic
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
Shapes and Positions of Black Hole Shadows in Accretion Disks and Spin Parameters of Black Holes
Can we determine a spin parameter of a black hole by observation of a black
hole shadow in an accretion disk? In order to answer this question, we make a
qualitative analysis and a quantitative analysis of a shape and a position of a
black hole shadow casted by a rotating black hole on an optically thick
accretion disk and its dependence on an angular momentum of a black hole. We
have found black hole shadows with a quite similar size and a shape for largely
different black hole spin parameters and a same black hole mass. Thus, it is
practically difficult to determine a spin parameter of a black hole from a size
and a shape of a black hole shadow in an accretion disk. We newly introduce a
bisector axis of a black hole shadow named a shadow axis. For a rotating black
hole a shape and a position of a black hole shadow are not symmetric with
respect to a rotation axis of a black hole shadow. So, in this case the minimum
interval between a mass center of a black hole and a shadow axis is finite. An
extent of this minimum interval is roughly proportional to a spin parameter of
a black hole for a fixed inclination angle between a rotation axis of a black
hole and a direction of an observer. In order to measure a spin parameter of a
black hole, if a shadow axis is determined observationally, it is crucially
important to determine a position of a mass center of a black hole in a region
of a black hole shadow.Comment: 13 pages, 6 figures, accepted for publication in Ap
Super-critical Accretion Flows around Black Holes: Two-dimensional, Radiation-pressure-dominated Disks with Photon-trapping
The quasi-steady structure of super-critical accretion flows around a black
hole is studied based on the two-dimensional radiation-hydrodynamical (2D-RHD)
simulations. The super-critical flow is composed of two parts: the disk region
and the outflow regions above and below the disk. Within the disk region the
circular motion as well as the patchy density structure are observed, which is
caused by Kelvin-Helmholtz instability and probably by convection. The
mass-accretion rate decreases inward, roughly in proportion to the radius, and
the remaining part of the disk material leaves the disk to form outflow because
of strong radiation pressure force. We confirm that photon trapping plays an
important role within the disk. Thus, matter can fall onto the black hole at a
rate exceeding the Eddington rate. The emission is highly anisotropic and
moderately collimated so that the apparent luminosity can exceed the Eddington
luminosity by a factor of a few in the face-on view. The mass-accretion rate
onto the black hole increases with increase of the absorption opacity
(metalicity) of the accreting matter. This implies that the black hole tends to
grow up faster in the metal rich regions as in starburst galaxies or
star-forming regions.Comment: 16 pages, 12 figures, accepted for publication in ApJ (Volume 628,
July 20, 2005 issue
Isothermal Shock Formation in Non-Equatorial Accretion Flows around Kerr Black Holes
We explore isothermal shock formation in non-equatorial, adiabatic accretion
flows onto a rotating black hole, with possible application to some active
galactic nuclei (AGNs). The isothermal shock jump conditions as well as the
regularity condition, previously developed for one-dimensional (1D) flows in
the equatorial plane, are extended to two-dimensional (2D), non-equatorial
flows, to explore possible geometrical effects. The basic hydrodynamic
equations with these conditions are self-consistently solved in the context of
general relativity to explore the formation of stable isothermal shocks. We
find that strong shocks are formed in various locations above the equatorial
plane, especially around a rapidly-rotating black hole with the prograde flows
(rather than a Schwarzschild black hole). The retrograde flows are generally
found to develop weaker shocks. The energy dissipation across the shock in the
hot non-equatorial flows above the cooler accretion disk may offer an
attractive illuminating source for the reprocessed features, such as the iron
fluorescence lines, which are often observed in some AGNs.Comment: 22 pages with 11 figures, presented at 5th international conference
on high energy density laboratory astrophysics in Tucson, Arizona. accepted
to Ap
General Relativistic Hydrodynamic Simulations and Linear Analysis of the Standing Accretion Shock Instability around a Black Hole
We study the stability of standing shock waves in advection-dominated
accretion flows into a Schwarzschild black hole by 2D general relativistic
hydrodynamic simulations as well as linear analysis in the equatorial plane. We
demonstrate that the accretion shock is stable against axisymmetric
perturbations but becomes unstable to non-axisymmetric perturbations. The
results of dynamical simulations show good agreement with linear analysis on
the stability, oscillation and growing time scales. The comparison of different
wave-travel times with the growth time scales of the instability suggests that
the instability is likely to be of the Papaloizou-Pringle type, induced by the
repeated propagations of acoustic waves. However, the wavelengths of
perturbations are too long to clearly define the reflection point. By analyzing
the non-linear phase in the dynamical simulations, it is shown that quadratic
mode couplings precede the non-linear saturation. It is also found that not
only short-term random fluctuations by turbulent motions but also quasi
periodic oscillations take place on longer time scales in the non-linear phase.
We give some possible implications of the instability for quasi periodic
oscillations (QPOs) and the central engine for gamma ray bursts (GRBs).Comment: 34 pages, 11 figures, accepted in Ap
The role of flow geometry in influencing the stability criteria for low angular momentum axisymmetric black hole accretion
Using mathematical formalism borrowed from dynamical systems theory, a
complete analytical investigation of the critical behaviour of the stationary
flow configuration for the low angular momentum axisymmetric black hole
accretion provides valuable insights about the nature of the phase trajectories
corresponding to the transonic accretion in the steady state, without taking
recourse to the explicit numerical solution commonly performed in the
literature to study the multi-transonic black hole accretion disc and related
astrophysical phenomena. Investigation of the accretion flow around a non
rotating black hole under the influence of various pseudo-Schwarzschild
potentials and forming different geometric configurations of the flow structure
manifests that the general profile of the parameter space divisions describing
the multi-critical accretion is roughly equivalent for various flow geometries.
However, a mere variation of the polytropic index of the flow cannot map a
critical solution from one flow geometry to the another, since the numerical
domain of the parameter space responsible to produce multi-critical accretion
does not undergo a continuous transformation in multi-dimensional parameter
space. The stationary configuration used to demonstrate the aforementioned
findings is shown to be stable under linear perturbation for all kind of flow
geometries, black hole potentials, and the corresponding equations of state
used to obtain the critical transonic solutions. Finally, the structure of the
acoustic metric corresponding to the propagation of the linear perturbation
studied are discussed for various flow geometries used.Comment: 13 pages. 5 figure
Mass Outflow Rate From Accretion Discs around Compact Objects
We compute mass outflow rates from accretion disks around compact objects,
such as neutron stars and black holes. These computations are done using
combinations of exact transonic inflow and outflow solutions which may or may
not form standing shock waves. Assuming that the bulk of the outflow is from
the effective boundary layers of these objects, we find that the ratio of the
outflow rate and inflow rate varies anywhere from a few percent to even close
to a hundred percent (i.e., close to disk evacuation case) depending on the
initial parameters of the disk, the degree of compression of matter near the
centrifugal barrier, and the polytropic index of the flow. Our result, in
general, matches with the outflow rates obtained through a fully time-dependent
numerical simulation. In some region of the parameter space when the standing
shock does not form, our results indicate that the disk may be evacuated and
may produce quiescence states.Comment: 30 Latex pages and 13 figures. crckapb.sty; Published in Class.
Quantum Grav. Vol. 16. No. 12. Pg. 387
An Analytical Study on the Multi-critical Behaviour and Related Bifurcation Phenomena for Relativistic Black Hole Accretion
We apply the theory of algebraic polynomials to analytically study the
transonic properties of general relativistic hydrodynamic axisymmetric
accretion onto non-rotating astrophysical black holes. For such accretion
phenomena, the conserved specific energy of the flow, which turns out to be one
of the two first integrals of motion in the system studied, can be expressed as
a 8 degree polynomial of the critical point of the flow configuration.
We then construct the corresponding Sturm's chain algorithm to calculate the
number of real roots lying within the astrophysically relevant domain of
. This allows, for the first time in literature, to {\it
analytically} find out the maximum number of physically acceptable solution an
accretion flow with certain geometric configuration, space-time metric, and
equation of state can have, and thus to investigate its multi-critical
properties {\it completely analytically}, for accretion flow in which the
location of the critical points can not be computed without taking recourse to
the numerical scheme. This work can further be generalized to analytically
calculate the maximal number of equilibrium points certain autonomous dynamical
system can have in general. We also demonstrate how the transition from a
mono-critical to multi-critical (or vice versa) flow configuration can be
realized through the saddle-centre bifurcation phenomena using certain
techniques of the catastrophe theory.Comment: 19 pages, 2 eps figures, to appear in "General Relativity and
Gravitation
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