The radial–azimuthal instability of accretion disk with anomalous viscosity

2014 ◽  
Vol 92 (5) ◽  
pp. 395-400
Author(s):  
Yue Qi Chen ◽  
Wei Qun Jiang
Keyword(s):  
Numerical Simulations ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Radiation Pressure ◽  
Gas Pressure ◽  
Thermal Mode ◽  
Acoustic Modes ◽  
Anomalous Viscosity ◽  
The Stability ◽  
Inner Region

The stability of the accretion disk is solved by numerical simulations when the radial and azimuthal perturbations are considered, where we adopt the anomalous viscosity model, which is close to real accretion disks. The results are discussed in the inner, intermediate, and outer regions of the accretion disk, respectively. With the increase of viscosity, α, the thermal mode and the viscous mode, as well as the acoustic modes, become more unstable in the disk dominated by radiation pressure (inner region). The instability is also influenced by the azimuthal perturbation wavenumber, n. With the increase of n, the thermal mode becomes more unstable, while the in-mode and out-mode become more stable no matter if the disk is dominated by radiation pressure or by gas pressure (intermediate and outer regions). There are many differences between our results and others’ results, especially in the inner region of the disk, when the anomalous viscosity is considered.

scholarly journals Dynamo Action in Accretion Disks

1993 ◽  
Vol 157 ◽  
pp. 209-210
Author(s):  
Ulf Torkelsson
Keyword(s):  
Numerical Simulations ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Standard Theory ◽  
Dynamo Action ◽  
Preliminary Results

Employing the standard theory for thin accretion disks I estimate the relevant parameters for a dynamo in an accretion disk. These estimates could then be compared to the results of numerical simulations. Some preliminary results of such simulations (Torkelsson & Brandenburg 1992) are presented too.

scholarly journals Accretion disks around a mass with quadrupole QUADRUPOLE

2021 ◽  
Vol 79 (1) ◽  
pp. 352-358
Author(s):  
S. Toktarbay ◽  
◽  
A.Zh. Abylaeva ◽  
G.N. Khudaibergenova ◽  
B.S. Nasyrova ◽  
...  
Keyword(s):  
Accretion Disk ◽  
Accretion Disks ◽  
Test Particle ◽  
Compact Objects ◽  
Geodesic Line ◽  
Field Equations ◽  
Schwarzschild Solution ◽  
Test Particles ◽  
Line Equation ◽  
The Stability

In this work, we consider the exterior static axisymmetric gravitational of compact objects. We investigate the properties of the q-metric which is the simplest generalization of the Schwarzschild solution that contains a quadrupole parameter. The geodesic line equation is derived from the field equations and the orbits of the test particle are investigated. We consider the stability properties of test particles moving along circular orbits around a mass with quadrupole. We show that the quadrupole modifies drastically the properties of an accretion disk made of such test particles.

scholarly journals Hybrid Accretion-Disks in AGN and the AGN Contribution to the XRB

1994 ◽  
Vol 159 ◽  
pp. 491-491
Author(s):  
Amri Wandel
Keyword(s):  
Accretion Disk ◽  
Accretion Disks ◽  
Power Law ◽  
Accretion Rate ◽  
Bremsstrahlung Spectrum ◽  
Spectral Evolution ◽  
X Ray ◽  
X Ray Background ◽  
Inner Region ◽  
Two Temperature

The hybrid accretion-disk (HAD) model links the two characteristic components of AGN spectra – the UV bump and the X-ray power-law – in the framework of one physical model. The radially stratified hybrid disk is a self consistent combination of a thin, cool accretion disk at large radii, with an inner hot two-temperature disk. Its spectrum consists of three components, corresponding to the three radial disk regions: a blackbody thermal spectrum from the outer cool disk, a Comptonized soft photon power-law spectrum from the intermediate region, and a thermal Comptonized bremsstrahlung spectrum from the inner region. The dependence of the hybrid disk spectrum on the accretion rate and on other parameters is discussed and applied to AGN spectral evolution, and in particular to explaining the cosmic X-ray background by AGN.

scholarly journals Form of a microlensed line from accretion disk in the linear caustic approximation

2018 ◽  
pp. 27-31
Author(s):  
V. Zhdanov ◽  
E. Fedorova ◽  
M. Khelashvili
Keyword(s):  
Black Hole ◽  
Spectral Line ◽  
Numerical Simulations ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Gravitational Lens ◽  
Emission Lines ◽  
Line Profiles ◽  
Lens System ◽  
Central Black Hole

The line profiles like that of the fluorescent Fe K or Co K lines in the X-ray spectra of the active galactic nuclei (AGN) reflect characteristics of the central regions of these objects. These lines can be formed in the accretion disks around central supermassive black holes and their shapes are connected with the central black hole spin and the accretion disk inclination angle to the line-of-the-sight. If an AGN is a source of a gravitational lens system with microlensing events, one can get an additional important information about both the accretion disk parameters and gravitational lens parameters as well. Microlensing processes were observed in such gravitational lens systems, as PKS 1830-211, B0218+357, RX J1131-1231 i HE1104-1805, Q2237+0305 and we can suspect to observe there also the spectral appearances of microlensing. Here we performed the numerical simulations of the microlensed relativistic spectral line profiles formed in the AGN accretion disks. Using the inear caustic model we show that the time dependence of the profile is determined essentially by the angle between to the disk axis and the caustic. This gives us an opportunity to assess this orientation. Microlens caustics magnify some parts of the accretion disk more prominently than others. Due to the Doppler effects and differences in the rotation direction this leads to the frequency-dependent magnification which distorts the profile of a relativistic spectral line. Such deformations are variable with time due to relative motions of the source and the microlens, and they can give us possibility to obtain some additional information about the disk brightness profile and caustic orientation relatively to the disk. Here we consider the thin disk model, Schwarzschild black hole, and the linear caustic approximation as well. The numerical simulations of the relativistic emission line profiles distorted by strong gravitational microlensing effect were performed for several different orientations of the linear caustic relatively to the disk, as well as several inclinations of the disk to the line-of-the-sight. Basic presumptions for the numerical modeling were the following: (a) AGN is a source in the gravitational lens system and it its inner parts the luminescent emission lines with relativistic profiles are being emitted; (b) this line is formed in the thin accretion disk quite far away from the central black hole and can be calculated with no taking into account the relativistic effects; (c) the caustic can be considered as a linear one. We show that the relative orientation of the caustic and the disk can be determined from emission lines profiles. Our numerical simulations demonstrate that the difference between profiles corresponding to different caustic orientations appears to be more prominent during the first half of the strong microlensing event, namely, before the crossing the disk center, and this dependence is irrespective to the accretion disk brightness profile. We show that for the spectral accuracy level high enough we have a perspective to determine the caustic orientation from the observational data.

The Contribution of Degenerate Electron Pressure to The Stability of The Outer Region of Thin Keplerian Accretion Disks Around a Neutron Star

2015 ◽  
Vol 11 (1) ◽  
pp. 2886-2891
Author(s):  
Abbi Seyoum Demissie
Keyword(s):  
Neutron Star ◽  
Accretion Disks ◽  
Outer Region ◽  
Thin Disk ◽  
Gas Pressure ◽  
Degenerate Electron ◽  
Small Temperature ◽  
Radial Perturbation ◽  
The Stability ◽  
Definite Temperature

The stability analysis of a geometrically thin, gas-pressure dominated accretion disk around a neutron star is presented. In purely radial perturbation case, thin disk is stable to thermal modes. The stability is analyzed at a small temperature, that is temperature approaching zero and at definite temperature. The contribution of both fully and partially degenerate electrons pressure for the stability of the disk in its outer region is investigated. We find that the disk is stable in this region, where the gas pressure is more dominant than radiation pressure.

scholarly journals Radiative Winds from Accretion Disks in CVS

1998 ◽  
Vol 188 ◽  
pp. 348-349
Author(s):  
M. Hachiya ◽  
Y. Tajima ◽  
J. Fukue
Keyword(s):  
Boundary Layers ◽  
Accretion Disk ◽  
Wind Velocity ◽  
Accretion Disks ◽  
White Dwarf ◽  
Escape Velocity ◽  
Bipolar Outflows ◽  
X Ray ◽  
Inner Radius ◽  
Inner Region

P Cyg profiles observed in CVs strongly suggest the existence of accretion disk winds in CVs. Recently, X-ray observations revealed bipolar outflows in supersoft X-ray sources. The wind velocity measured in these observations is about 3000-5000 km s−1, which is of the order of the escape velocity of white dwarfs. Hence, it is supposed that the winds would originate from the inner disk and/or the boundary layers between the disk and white dwarf. We examine radiatively-acclerated accretion-disk winds, and found that the terminal speed of the wind emanated from the inner region becomes ~ 0.6GM/rin, where M is the white dwarf mass and rin the inner radius of the disk.

scholarly journals The star capture model for fueling quasar accretion disks

2010 ◽  
Vol 6 (S271) ◽  
pp. 381-382 ◽  
Author(s):  
Gareth F. Kennedy ◽  
Jordi Miralda-Escudé ◽  
Juna A. Kollmeier
Keyword(s):  
Black Hole ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Thin Disk ◽  
Central Black Hole ◽  
Zone Of Influence ◽  
Inner Region

AbstractAlthough the powering mechanism for quasars is now widely recognized to be the accretion of matter in a geometrically thin disk, the transport of matter to the inner region of the disk where luminosity is emitted remains an unsolved question. Miralda-Escudé & Kollmeier (2005) proposed a model whereby quasars are fuelled when stars are captured by the accretion disk as they plunge through the gas. Such plunging stars can then be destroyed and deliver their mass to the accretion disk.Here we present the first detailed calculations for the capture of stars originating far from the accretion disk near the zone of influence of the central black hole. In particular we examine the effect of adding a perturbing mass to a fixed stellar cusp potential on bringing stars into the accretion disk where they can be captured. The work presented here will be discussed in detail in an upcoming publication Kennedy et al. (2010).

scholarly journals Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks

1997 ◽  
Vol 163 ◽  
pp. 311-320 ◽  
Author(s):  
Philip R. Maloney ◽  
Mitchell C. Begelman
Keyword(s):  
Active Galactic Nuclei ◽  
Accretion Disk ◽  
Accretion Disks ◽  
Radiation Pressure ◽  
Global Analysis ◽  
Galactic Nuclei ◽  
Local Analysis ◽  
X Ray ◽  
Ss 433 ◽  
X Ray Binaries

AbstractA geometrically thin, optically thick, warped accretion disk with a central source of luminosity is subject to non-axisymmetric forces due to radiation pressure; the resulting torque acts to modify the warp. Initially planar accretion disks are unstable to warping driven by radiation torque, as shown in a local analysis by Pringle (1996) and a global analysis of the stable and unstable modes by Maloney, Begelman, & Pringle (1996). In general, the warp also precesses.We discuss the nature of this instability, and its possible implications for accretion disks in X-ray binaries and active galactic nuclei. Specifically, we argue that this effect provides a plausible explanation for the misalignment and precession of the accretion disks in X-ray binaries such as SS 433 and Her X–l; the same mechanism explains why the maser disk in NGC 4258 is warped.

The structure and stability of accretion disks surrounding black holes with anomalous viscosity

2007 ◽  
Vol 73 (2) ◽  
pp. 273-283 ◽  
Author(s):  
YUEQI CHEN ◽  
SANQIU LIU
Keyword(s):  
Growth Rate ◽  
Black Holes ◽  
Accretion Disks ◽  
Magnetic Viscosity ◽  
Anomalous Viscosity ◽  
Outer Disk ◽  
Rotating Black Holes ◽  
Special Value ◽  
Local Sound ◽  
The Stability

Abstract.The solution of the steady state of accretion disks surrounding non-rotating black holes is solved by numerical simulations, where we adopt a promising viscosity prescription (i.e. anomalous magnetic viscosity). We can describe the disk more exactly than other studies, because the viscosity we adopted is close to the truth accretion disk. In contrast to previous studies, the curve of the Mach number behaves differently owing to the decline of the local sound velocity with increasing radii.The stability of these disks has been examined. We find that the O-mode (out-mode) is always unstable and its growth rate decreases monotonically with increasing wavelength, in the inner disk and in the outer disk. The I-mode (in-mode) is unstable in the outer disk, but stable in the inner disk when the wavelength is greater than a special value.

Saturation mechanism and generated viscosity in gravito-turbulent accretion disks

2020 ◽  
Vol 640 ◽  
pp. A53
Author(s):  
L. Löhnert ◽  
S. Krätschmer ◽  
A. G. Peeters
Keyword(s):  
Growth Rate ◽  
Numerical Simulations ◽  
Accretion Disks ◽  
Gravitational Instability ◽  
Turbulent Viscosity ◽  
Radial Distance ◽  
Maximum Growth ◽  
Wave Number ◽  
Radiative Cooling ◽  
Mixing Length

Here, we address the turbulent dynamics of the gravitational instability in accretion disks, retaining both radiative cooling and irradiation. Due to radiative cooling, the disk is unstable for all values of the Toomre parameter, and an accurate estimate of the maximum growth rate is derived analytically. A detailed study of the turbulent spectra shows a rapid decay with an azimuthal wave number stronger than ky−3, whereas the spectrum is more broad in the radial direction and shows a scaling in the range kx−3 to kx−2. The radial component of the radial velocity profile consists of a superposition of shocks of different heights, and is similar to that found in Burgers’ turbulence. Assuming saturation occurs through nonlinear wave steepening leading to shock formation, we developed a mixing-length model in which the typical length scale is related to the average radial distance between shocks. Furthermore, since the numerical simulations show that linear drive is necessary in order to sustain turbulence, we used the growth rate of the most unstable mode to estimate the typical timescale. The mixing-length model that was obtained agrees well with numerical simulations. The model gives an analytic expression for the turbulent viscosity as a function of the Toomre parameter and cooling time. It predicts that relevant values of α = 10−3 can be obtained in disks that have a Toomre parameter as high as Q ≈ 10.

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