18 research outputs found
Resistive and magnetized accretion flows with convection
We considered the effects of convection on the radiatively inefficient
accretion flows (RIAF) in the presence of resistivity and toroidal magnetic
field. We discussed the effects of convection on transports of angular momentum
and energy. We established two cases for the resistive and magnetized RIAFs
with convection: assuming the convection parameter as a free parameter and
using mixing-length theory to calculate convection parameter. A self-similar
method was used to solve the integrated equations that govern the behavior of
the presented model. The solutions showed that the accretion and rotational
velocities decrease by adding the convection parameter, while the sound speed
increases. Moreover, by using mixing-length theory to calculate convection
parameter, we found that the convection can be important in RIAFs with magnetic
field and resistivity.Comment: 7 pages, 3 figures, accepted by Ap&S
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
Low-Luminosity Accretion in Black Hole X-ray Binaries and Active Galactic Nuclei
At luminosities below a few percent of Eddington, accreting black holes
switch to a hard spectral state which is very different from the soft
blackbody-like spectral state that is found at higher luminosities. The hard
state is well-described by a two-temperature, optically thin, geometrically
thick, advection-dominated accretion flow (ADAF) in which the ions are
extremely hot (up to K near the black hole), the electrons are also
hot ( K), and thermal Comptonization dominates the X-ray
emission. The radiative efficiency of an ADAF decreases rapidly with decreasing
mass accretion rate, becoming extremely low when a source reaches quiescence.
ADAFs are expected to have strong outflows, which may explain why relativistic
jets are often inferred from the radio emission of these sources. It has been
suggested that most of the X-ray emission also comes from a jet, but this is
less well established.Comment: To appear in "From X-ray Binaries to Quasars: Black Hole Accretion on
All Mass Scales" edited by T. Maccarone, R. Fender, L. Ho, to be published as
a special edition of "Astrophysics and Space Science" by Kluwe
The interaction of dark matter cusp with the baryon component in disk galaxies
In this paper we examine the effect of the formation and evolution of the
disk galaxy on the distribution of dark halo matter. We have made simulations
of isolated dark matter (DM) halo and two component (DM + baryons). N-body
technique was used for stellar and DM particles and TVD MUSCL scheme for
gas-dynamic simulations. The simulations include the processes of star
formation, stellar feedback, heating and cooling of the interstellar medium.
The results of numerical experiments with high spatial resolution let us to
conclude in two main findings. First, accounting of star formation and
supernova feedback resolves the so-called problem of cusp in distribution of
dark matter predicted by cosmological simulations. Second, the interaction of
dark matter with dynamic substructures of stellar and gaseous galactic disk
(e.g., spiral waves, bar) has an impact on the shape of the dark halo. In
particular, the in-plane distribution of dark matter is more symmetric in runs,
where the baryonic component was taken into account.Comment: 7 pages, 6 figure
Dynamics of charged dust particles in protoplanetary discs
We study the effect of an imposed magnetic field on the motion of charged
dust particles in magnetically active regions of a protoplanetary disc.
Assuming a power law structure for the vertical and the toroidal components of
the magnetic field for the regions beyond magnetically dead region of the disc,
the radial and the vertical velocities of the charged particles, in the
asymptotic case of small particles, are calculated analytically. While grains
with radii smaller than a critical radius significantly are affected by the
magnetic force, motion of the particles with larger radii is independent of the
magnetic field. The critical radius depends on the magnetic geometry and the
charge of the grains. Assuming that a grain particle has one elementary charge
and the physical properties of the disc correspond to a minimum-mass solar
nebula, we show that only micron-sized grains are affected by the magnetic
force. Also, charge polarity determines direction of the radial velocity. For
such small particles, both the radial and the vertical velocities increase due
to the magnetic force.Comment: Accepted for publication in Astrophysics & Space Scienc
Current Status of Simulations
As the title suggests, the purpose of this chapter is to review the current
status of numerical simulations of black hole accretion disks. This chapter
focuses exclusively on global simulations of the accretion process within a few
tens of gravitational radii of the black hole. Most of the simulations
discussed are performed using general relativistic magnetohydrodynamic (MHD)
schemes, although some mention is made of Newtonian radiation MHD simulations
and smoothed particle hydrodynamics. The goal is to convey some of the exciting
work that has been going on in the past few years and provide some speculation
on future directions.Comment: 15 pages, 14 figures, to appear in the proceedings of the ISSI-Bern
workshop on "The Physics of Accretion onto Black Holes" (8-12 October 2012
Theory of magnetically powered jets
The magnetic theory for the production of jets by accreting objects is
reviewed with emphasis on outstanding problem areas. An effort is made to show
the connections behind the occasionally diverging nomenclature in the
literature, to contrast the different points of view about basic mechanisms,
and to highlight concepts for interpreting the results of numerical
simulations. The role of dissipation of magnetic energy in accelerating the
flow is discussed, and its importance for explaining high Lorentz factors. The
collimation of jets to the observed narrow angles is discussed, including a
critical discussion of the role of `hoop stress'. The transition between disk
and outflow is one of the least understood parts of the magnetic theory; its
role in setting the mass flux in the wind, in possible modulations of the mass
flux, and the uncertainties in treating it realistically are discussed. Current
views on most of these problems are still strongly influenced by the
restriction to 2 dimensions (axisymmetry) in previous analytical and numerical
work; 3-D effects likely to be important are suggested. An interesting problem
area is the nature and origin of the strong, preferably highly ordered magnetic
fields known to work best for jet production. The observational evidence for
such fields and their behavior in numerical simulations is discussed. I argue
that the presence or absence of such fields may well be the `second parameter'
governing not only the presence of jets but also the X-ray spectra and timing
behavior of X-ray binaries.Comment: 29 pages. Publication delays offered the opportunity for further
corrections, an expansion of sect 4.2, and one more Fig. To appear in
Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect.
Notes Phys. 794 (2009
A possible influence on standard model of quasars and active galactic nuclei in strong magnetic field
Polar direct drive – Ignition at 1 MJ
Target designs to achieve direct-drive ignition on the
NIF using the x-ray-drive beam configuration are examined. This approach,
known as polar direct drive (PDD), achieves the required irradiation
uniformity by repointing some of the beams toward the target equator, and by
increasing the laser intensity at the equator to compensate for the reduced
laser coupling from oblique irradiation. Techniques to increase the
equatorial intensity can include using phase plates that produce elliptical
spot shapes, increasing the power in beams directed toward the equator, and
using a ring offset from the equator to redirect rays toward the target
normal. The requirements for beam pointing, power balance, single-beam
smoothing, and inner-ice-surface roughness are examined. Designs with an
incident laser energy of 1.0 MJ are presented. The simulations use the 2-D
hydrocode DRACO with 3-D ray trace to model the laser irradiation and Monte Carlo
alpha particle transport to model the thermonuclear burn