2,648 research outputs found
A Note on Bimodal Accretion Disks
The existence of bimodal disks is investigated. Following a simple argument
based on energetic considerations we show that stationary, bimodal accretion
disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an
advection dominated accretion flow (ADAF) at smaller radii are never possible
using the standard slim disk approach, unless some extra energy flux is
present. The same argument, however, predicts the possibility of a transition
from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE
disk to a SSD. Both types of solutions have been found.Comment: 9 pages including 9 figures, accepted for publication in The
Astrophysical Journa
Shakura-Sunyaev Disk Can Smoothly Match Advection-Dominated Accretion Flow
We use the standard Runge-Kutta method to solve the set of basic equations
describing black hole accretion flows composed of two-temperature plasma. We do
not invoke any extra energy transport mechanism such as thermal conduction and
do not specify any ad hoc outer boundary condition for the advection-dominated
accretion flow (ADAF) solution. We find that in the case of high viscosity and
non-zero radiative cooling, the ADAF solution can have an asymptotic approach
to the Shakura-Sunyaev disk (SSD) solution, and the SSD-ADAF transition radius
is close to the central black hole. Our results further prove the mechanism of
thermal instability-triggered SSD-ADAF transition suggested previously by
Takeuchi & Mineshige and Gu & Lu.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter
Criterion for Generation of Winds from Magnetized Accretion Disks
An analytic model is proposed for non-radiating accretion flows accompanied
by up or down winds in a global magnetic field. Physical quantities in this
model solution are written in variable-separated forms, and their radial parts
are simple power law functions including one parameter for wind strength.
Several, mathematically equivalent but physically different expressions of the
criterion for wind generation are obtained. It is suggested also that the
generation of wind is a consequence of the intervention of some mechanism that
redistributes the locally available gravitational energy, and that the
Bernoulli sum can be a good indicator of the existence of such mechanisms.Comment: 24 pages, 0 figures, ApJ accepte
Self-Similar Accretion Flows with Convection
We consider height-integrated equations of an advection-dominated accretion
flow (ADAF), assuming that there is no mass outflow. We include convection
through a mixing length formalism. We seek self-similar solutions in which the
rotational velocity and sound speed scale as R^{-1/2}, where R is the radius,
and consider two limiting prescriptions for the transport of angular momentum
by convection. In one limit, the transport occurs down the angular velocity
gradient, so convection moves angular momentum outward. In the other, the
transport is down the specific angular momentum gradient, so convection moves
angular momentum inward. We also consider general prescriptions which lie in
between the two limits.
When convection moves angular momentum outward, we recover the usual
self-similar solution for ADAFs in which the mass density scales as rho ~
R^{-3/2}. When convection moves angular momentum inward, the result depends on
the viscosity coefficient alpha. If alpha>alpha_{crit1} ~ 0.05, we once again
find the standard ADAF solution. For alpha<alpha_{crit}, however, we find a
non-accreting solution in which rho ~ R^{-1/2}. We refer to this as a
"convective envelope" solution or a "convection-dominated accretion flow".
Two-dimensional numerical simulations of ADAFs with values of alpha<0.03 have
been reported by several authors. The simulated ADAFs exhibit convection. By
virtue of their axisymmetry, convection in these simulations moves angular
momentum inward, as we confirm by computing the Reynolds stress. The
simulations give rho ~ R^{-1/2}, in good agreement with the convective envelope
solution. The R^{-1/2} density profile is not a consequence of mass outflow.Comment: 22 pages, 4 figures, final version accepted for publication in ApJ, a
new appendix was added and 3 figs were modifie
High-growth-rate magnetohydrodynamic instability in differentially rotating compressible flow
The transport of angular momentum in the outward direction is the fundamental
requirement for accretion to proceed in an accretion disc. This objective can
be achieved if the accretion flow is turbulent. Instabilities are one of the
sources for the turbulence. We study a differentially rotating compressive flow
in the presence of non vanishing radial and azimuthal magnetic field and
demonstrate the occurrence of a high growth rate instability. This instability
operates in a region where magnetic energy density exceeds the rotational
energy density
Spectral Models of Convection-Dominated Accretion Flows
For small values of the dimensionless viscosity parameter, namely
, the dynamics of non-radiating accretion flows is
dominated by convection; convection strongly suppresses the accretion of matter
onto the central object and transports a luminosity from small to large radii in the flow. A fraction of this convective
luminosity is likely to be radiated at large radii via thermal bremsstrahlung
emission. We show that this leads to a correlation between the frequency of
maximal bremsstrahlung emission and the luminosity of the source, . Accreting black holes with X-ray luminosities are expected to
have hard X-ray spectra, with photon indices , and sources with
are expected to have soft spectra, with
. This is testable with {\it Chandra} and {\it XMM}.Comment: final version accepted by ApJ; significant modifications from
previous versio
Distance Measurement of Galaxies to Redshift of 0.1 using the CO-Line Tully-Fisher Relation
We report on the first results of a long-term project to derive distances of
galaxies at cosmological distances by applying the CO-line width-luminosity
relation. We have obtained deep CO-line observations of galaxies at redshifts
up to 29,000 km/s using the Nobeyama 45-m mm-wave Telescope, and some
supplementary data were obtained by using the IRAM 30-m telescope. We have
detected the CO line emission for several galaxies, and used their CO line
widths to estimate the absolute luminosities using the line-width-luminosity
relation. In order to obtain photometric data and inclination correction, we
also performed optical imaging observations of the CO-detected galaxies using
the CFHT 3.6-m telescope at high resolution. The radio and optical data have
been combined to derive the distance moduli and distances of the galaxies, and
Hubble ratios were estimated for these galaxies. We propose that the CO line
width-luminosity relation can be a powerful method to derive distances of
galaxies to redfhift of z = 0.1 and to derive the Hubble ratio in a significant
volume of the universe.
Key words: Cosmology - Galaxies: general - Distance scale - CO lineComment: To appear in PASJ, Plain Tex, 3 figures (in 10 ps files
Advection-dominated Inflow/Outflows from Evaporating Accretion Disks
In this Letter we investigate the properties of advection-dominated accretion
flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD).
In our picture the ADAF fills the central cavity evacuated by the SSD and
extends beyond the transition radius into a coronal region. We find that,
because of global angular momentum conservation, a significant fraction of the
hot gas flows away from the black hole forming a transsonic wind, unless the
injection rate depends only weakly on radius (if , ). The Bernoulli number of the inflowing gas is negative
if the transition radius is Schwarzschild radii, so matter
falling into the hole is gravitationally bound. The ratio of inflowing to
outflowing mass is , so in these solutions the accretion rate is
of the same order as in standard ADAFs and much larger than in
advection-dominated inflow/outflow models (ADIOS). The possible relevance of
evaporation-fed solutions to accretion flows in black hole X-ray binaries is
briefly discussed.Comment: 5 pages Latex with 2 ps figures. Accepted for publication in ApJ
Letter
Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579
M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei
which have an estimated mass for the central black hole, detected hard X-ray
emission, and detected optical/UV emission. In contrast to the canonical ``big
blue bump,'' both have optical/UV spectra which decrease with increasing
frequency in a plot. Barring significant reddening by dust and/or
large errors in the black hole mass estimates, the optical/UV spectra of these
systems require that the inner edge of a geometrically thin, optically thick,
accretion disk lies at roughly 100 Schwarzschild radii. The observed X-ray
radiation can be explained by an optically thin, two temperature,
advection-dominated accretion flow at smaller radii.Comment: emulateapj.sty, to appear in ApJ Letter
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