153 research outputs found
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
Slim accretion discs: a model for ADAF-SLE transitions
We numerically construct slim, global, vertically integrated models of
optically thin, transonic accretion discs around black holes, assuming a
regularity condition at the sonic radius and boundary conditions at the outer
radius of the disc and near the black hole. In agreement with several previous
studies, we find two branches of shock-free solutions, in which the cooling is
dominated either by advection, or by local radiation. We also confirm that the
part of the accretion flow where advection dominates is in some circumstances
limited in size: it does not extend beyond a certain outer limiting radius. New
results found in our paper concern the location of the limiting radius and
properties of the flow near to it. In particular, we find that beyond the
limiting radius, the advective dominated solutions match on to Shapiro,
Lightman & Eardley (SLE) discs through a smooth transition region. Therefore,
the full global solutions are shock-free and unlimited in size. There is no
need for postulating an extra physical effect (e.g. evaporation) for triggering
the ADAF-SLE transition. It occurs due to standard accretion processes
described by the classic slim disc equations.Comment: 12 pages, 7 figures, MNRAS accepte
Convection-Dominated Accretion Flows
Non-radiating, advection-dominated, accretion flows are convectively
unstable. We calculate the two-dimensional (r-theta) structure of such flows
assuming that (1) convection transports angular momentum inwards, opposite to
normal viscosity and (2) viscous transport by other mechanisms (e.g., magnetic
fields) is weak (alpha << 1). Under such conditions convection dominates the
dynamics of the accretion flow and leads to a steady state structure that is
marginally stable to convection. We show that the marginally stable flow has a
constant temperature and rotational velocity on spherical shells, a net flux of
energy from small to large radii, zero net accretion rate, and a radial density
profile proportional to r^{-1/2}, flatter than the r^{-3/2} profile
characteristic of spherical accretion flows. This solution accurately describes
the full two-dimensional structure of recent axisymmetric numerical simulations
of advection-dominated accretion flows.Comment: final version accepted by ApJ; discussion expanded, references adde
The Magnetohydrodynamics of Convection-Dominated Accretion Flows
Radiatively inefficient accretion flows onto black holes are unstable due to
both an outwardly decreasing entropy (`convection') and an outwardly decreasing
rotation rate (the `magnetorotational instability'; MRI). Using a linear
magnetohydrodynamic stability analysis, we show that long-wavelength modes are
primarily destabilized by the entropy gradient and that such `convective' modes
transport angular momentum inwards. Moreover, the stability criteria for the
convective modes are the standard Hoiland criteria of hydrodynamics. By
contrast, shorter wavelength modes are primarily destabilized by magnetic
tension and differential rotation. These `MRI' modes transport angular momentum
outwards. The convection-dominated accretion flow (CDAF) model, which has been
proposed for radiatively inefficient accretion onto a black hole, posits that
inward angular momentum transport and outward energy transport by
long-wavelength convective fluctuations are crucial for determining the
structure of the accretion flow. Our analysis suggests that the CDAF model is
applicable to a magnetohydrodynamic accretion flow provided the magnetic field
saturates at a sufficiently sub-equipartition value (plasma beta >> 1), so that
long-wavelength convective fluctuations can fit inside the accretion disk.
Numerical magnetohydrodynamic simulations are required to determine whether
such a sub-equipartition field is in fact obtained.Comment: 17 pages including 3 figures. Accepted for publication in ApJ. New
appendix and figure were added; some changes of the text were made in
response to the referee
Statistical theory of thermal instability
A new statistical approach is presented to study the thermal instability
process of optically thin unmagnetized plasma. In this approach the time
evolution of mass distribution function over temperature is calculated. This
function characterizes the statistical properties of the multiphase medium of
arbitrary spaced three-dimensional structure of arbitrary temperature
perturbations. We construct our theory under the isobarical condition (P=const
over space), which is satisfied in the short wavelength limit. The developed
theory is illustrated in the case of thermal instability of a slowly expanding
interstellar cloud. Numerical solutions of equations of the statistical theory
are constucted and compared with hydrodynamical solutions. The results of both
approaches are identical in the short wavelength range when the isobarity
condition is satisfied. Also the limits of applicability of the statistical
theory are estimated. The possible evolution of initial spectrum of
perturbations is discussed. The proposed theory and numerical models can be
relevant to the formation of the two-phases medium in the ~1pc region around
quasars. Then small warm (T~10000K) clouds are formed as the result of thermal
instability in an expanded gas fragment, which is a product of either a
star-star or star-accretion disk collision.Comment: 11 pages, 8 figures, submitted to MNRA
Two-dimensional models of hydrodynamical accretion flows into black holes
We present a systematic numerical study of two-dimensional axisymmetric
accretion flows around black holes. The flows have no radiative cooling and are
treated in the framework of the hydrodynamical approximation. The models
calculated in this study cover the large range of the relevant parameter space.
There are four types of flows, determined by the values of the viscosity
parameter and the adiabatic index : convective flows,
large-scale circulations, pure inflows and bipolar outflows. Thermal conduction
introduces significant changes to the solutions, but does not create a new flow
type. Convective accretion flows and flows with large-scale circulations have
significant outward-directed energy fluxes, which have important implications
for the spectra and luminosities of accreting black holes.Comment: 43 pages, 23 figures, submitted to Ap
Hard X-ray emitting black hole fed by accretion of low angular momentum matter
Observed spectra of Active Galactic Nuclei (AGN) and luminous X-ray binaries
in our Galaxy suggest that both hot (~10^9 K) and cold (~10^6 K) plasma
components exist close to the central accreting black hole. Hard X-ray
component of the spectra is usually explained by Compton upscattering of
optical/UV photons from optically thick cold plasma by hot electrons.
Observations also indicate that some of these objects are quite efficient in
converting gravitational energy of accretion matter into radiation. Existing
theoretical models have difficulties in explaining the two plasma components
and high intensity of hard X-rays. Most of the models assume that the hot
component emerges from the cold one due to some kind of instability, but no one
offers a satisfactory physical explanation for this. Here we propose a solution
to these difficulties that reverses what was imagined previously: in our model
the hot component forms first and afterward it cools down to form the cold
component. In our model, accretion flow has initially a small angular momentum,
and thus it has a quasi-spherical geometry at large radii. Close to the black
hole, the accreting matter is heated up in shocks that form due to the action
of the centrifugal force. The hot post-shock matter is very efficiently cooled
down by Comptonization of low energy photons and condensates into a thin and
cold accretion disk. The thin disk emits the low energy photons which cool the
hot component.Comment: 15 pages, 2 figures, submitted to ApJ Let
X-ray Images of Hot Accretion Flows
We consider the X-ray emission due to bremsstrahlung processes from hot, low
radiative-efficiency accretion flows around supermassive and galactic black
holes. We calculate surface brightness profiles and Michelson visibility
functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to
allow for the presence of outflows. We find that although the 1 keV emitting
region in these flows can always extend up to 10^6 Schwarzschild radii (R_S),
their surface brightness profiles and visibility functions are strongly
affected by the specific density profile. The advection-dominated solutions
with no outflows (p=0) lead to centrally peaked profiles with characteristic
sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to
flat intensity profiles with significantly larger characteristic sizes of up to
10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may
appear as extended X-ray sources when observed with current X-ray imaging
instruments. We show that X-ray brightness profiles and their associated
visibility functions may be powerful probes for determining the relevant mode
of accretion and, in turn, the properties of hot accretion flows. We discuss
the implications of our results for observations with the Chandra X-ray
Observatory and the planned X-ray interferometer MAXIM.Comment: 14 pages, 4 figures, accepted by The Astrophysical Journal, minor
change
Synchrotron Radiation From Radiatively Inefficient Accretion Flow Simulations: Applications to Sgr A*
We calculate synchrotron radiation in three-dimensional pseudo-Newtonian
magnetohydrodynamic simulations of radiatively inefficient accretion flows. We
show that the emission is highly variable at optically thin frequencies, with
order of magnitude variability on time-scales as short as the orbital period
near the last stable orbit; this emission is linearly polarized at the 20-50 %
level due to the coherent toroidal magnetic field in the flow. At optically
thick frequencies, both the variability amplitude and polarization fraction
decrease significantly with decreasing photon frequency. We argue that these
results are broadly consistent with the observed properties of Sgr A* at the
Galactic Center, including the rapid infrared flaring.Comment: Accepted for publication in Ap
Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows
We present three-dimensional MHD simulations of rotating radiatively
inefficient accretion flows onto black holes. In the simulations, we
continuously inject magnetized matter into the computational domain near the
outer boundary, and we run the calculations long enough for the resulting
accretion flow to reach a quasi-steady state. We have studied two limiting
cases for the geometry of the injected magnetic field: pure toroidal field and
pure poloidal field. In the case of toroidal field injection, the accreting
matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion
disk. The disk resembles in many respects the convection-dominated accretion
flows found in previous numerical and analytical investigations of viscous
hydrodynamic flows. Models with poloidal field injection evolve through two
distinct phases. In an initial transient phase, the flow forms a relatively
flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow.
However, when the flow later achieves steady state, it changes in character
completely. The magnetized accreting gas becomes two-phase, with most of the
volume being dominated by a strong dipolar magnetic field from which a thermal
low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating
radial streams which `diffuse' through the magnetic field with the help of
magnetic reconnection events.Comment: 35 pages including 3 built-in plots and 14 separate jpg-plots;
version accepted by Ap
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