Massive accretion disks

Recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging have revealed dense gas and dust accretion disks in nearby ultra-luminous galactic nuclei. In the best studied ultraluminousIR galaxy, Arp 220, the 2m imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gasmasses 10⁹M for each disk. The two gas disks in Arp 220 are counterrotating and their dynamical masses are 2×10⁹ M , that is, only slightly largerthan the gas masses. These disks have radii 100 pc and thickness 10-50 pc. The high brightness temperatures of the CO lines indicatethat the gas in the disks has area filling factors 25-50% and mean densitiesof 10⁴ cm^-3. Within these nuclear disks, the rate of massive star formation is undoubtedly prodigious and, given the high viscosity of the gas, there will also be high radial accretion rates, perhaps 10 M yr ^-1. If this inflow persists to very small radii, it is enough to feed even the highest luminosity AGNs.     

Magnetically tilted accretion disks

Accretion disks around magnetized, compact stars are expected to be tilted near their inner edges, due to the stresses exerted by the corotating magnetosphere of the inclined central rotator. We reassess numerically the results obtained analytically by Lipunovet al. (1981). Four qualitatively different situations occur, depending on the relative orientations of the outer accretion disk, the spin of the central rotator, and its magnetic dipole axis. In at least two of them, the inner part of the disk is expected to be decomposed into massive, magnetically confined clumps.     

Dissipation of thick accretion disks

A thick accretion disk which is isentropic cannot have simple laminar flow because fluid elements follow orbits which intersect the orbits of other fluid elements, leading to turbulence in astrophysical disks which have very large Reynolds numbers. The turbulence in such disks is estimated using molecular analogies for the behavior of the fluid elements. The usual empirical dissipation parameter ‘α’ is found to be equal to 0.25 under normal circumstances. Characteristic local disk parameters are calculated for a variety of conditions at different distances from a central star of one solar mass. Circumstances involving low midplane optical depths or external heating which can lead to large reductions in the turbulence are discussed.     

Magnetic fields of accretion disks and outflows in prograde and retrograde black holes

We show that for the accretion disk with equipartition between magnetic and radiative pressures, prograde black holes generate outflowing energy in jets more efficiently than retrograde black holes do. Both viscous radiative and irradiative disks provide more efficient outflow jets in the case of a prograde black hole than in the case of a retrograde black hole. Our results confirm the conclusion of Tchekhovskoy & McKinney (2012) that, for the same absolute value of the spin, prograde black holes with geometrically thick accretion disks generate outflows several times more efficiently than retrograde black holes do. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chaotic behavior in accretion disks

The eccentric luminosity variations of quasars are still a mystery. Analytic results of this behavior ranged from multi-periodic behavior to a purely random process. Recently, we have used the non-linear time-series analysis techniques to analyze the optical light curve of 3C 273 and found that its eccentric behavior may be a low-dimensional chaos. This result induces us to seek some non-linear mechanism for the eccentric luminosity variation. In this paper, we propose a simple non-linear accretion disk model and find that under some circumstances, the chaos appears both in the disk and in the light curve. Then we compute the outburst energy ΔI, defined by the difference between the maximum luminosity and the minimum luminosity, and the mean luminosity 〈I〉 and find that when chaos appears, ΔI～〈I〉 ^α ～M ^0.5α , where M is the mass of black hole and α≈1. These results are confirmed by and/or compatible with the observational data analysis.     

Theoretical restrictions on accretion disks

We construct a standard thin disk model taking into account the pressure from both gas and radiation, the opacity contributed by both electron scattering and absorption, and the gravity from both a central object and a disk. A simple and powerful technique for solving the non-linear equations is presented. Through a numerical algorithm for the two equationl for, , , all the disk quantities are expressed as the analytical function of, , . We also discuss the solutions in the limit cases 0, 1 and the parameter range of the linear approximation. From the numerical solutions and limit analyses, we found that it is not necessary to include the self-gravity of the disk.     

Meridional flow in thick accretion disks

We have studied the effect of the flow in the accretion disk. The specific angular momentum of the disk is assumed to be constant and the polytropic relation is used. We have solved the structure of the disk and the flow patterns of the irrotational perfect fluid.As far as the obtained results are concerned, the flow does not affect the shape of the configuration in the bulk of the disk, although the flow velocity reaches even a half of the sound velocity at the inner edge of the disk. Therefore, in order to study accretion disk models with the moderate mass accretion rate—i.e.,

Hydrodynamical activity in thin accretion disks

An asymptotic treatment of thin accretion disks, introduced by Kluźniak and Kita [Kluźniak, W., Kita, D., 2000. Three-dimensional structure of an alpha accretion disk. Available from: <arXiv:astro-ph/0006266v1> (KK)] for a steady-state disk flow, is extended to a time-dependent problem. Transient growth of axisymmetric disturbances is analytically shown to occur on the global disk scale. The implications of this result on the theory of hydrodynamical thin accretion disks, as well as future prospects, are discussed.     

Linkage between accretion disks and blazars

The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission region using a model for a relativistic axisymmetric jet combined with some simplifying assumptions based on the relativistic nature of the flow. The extrapolated magnetic field is compared with estimates based upon the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN 421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-Compton models are inconsistent with the magnetic fields extrapolated in this way. However, in two cases inverse Compton models in which a substantial part of the soft photon field is generated locally agree well, mainly because these models imply magnetic field strengths consistent with an important Poynting Flux component. This comparison is based on estimating the mass accretion rate from the jet energy flux. Further comparisons along these lines will be facilitated by independent estimates of the mass accretion rate in blazars and by more detailed models for jet propagation near the black hole.     

Relativistic reflection X-ray spectra of accretion disks

We have calculated the relativistic reflection component of the X-ray spectra of accretion disks in active galactic nuclei (AGN). Our calculations have shown that the spectra can be significantly modified by the motion of the accretion flow, and the gravity and rotation of the central black hole. The absorption edges in the spectra suffer severe en- ergy shifts and smearing, and the degree of distortion depends on the system parameters, in particular, the inner radius of the accretion disk and the disk viewing inclination angles. The effects are significant. Fluorescent X-ray emission lines from the inner accretion disk could be a powerful diagnostic of space-time distortion and dynamical relativistic effects near the event horizons of accreting black holes. However, improper treatment of the re- flection component in fitting the X-ray continuum could give rise to spurious line-like features. These features mimic the true fluorescent emission lines and may mask their relativistic signatures. Fully relativistic models for reflection continua together with the emission lines are needed in order to extract black-hole parameters from the AGN X-ray spectra.     

MHD simulations of jets from accretion disks

We present the MHD simulation including accretion flows in disks, acceleration of outflows from disks, and collimation of the outflows self-consistently. Although it was considered that this kind of simulations only shows the transient phenomena of jets, we found that the outflow and accretion flow reached a quasi-steady state by performing a long-term calculation in a large calculation region. Though the final stage is not exactly the steady state, the acceleration and collimation mechanisms of the outflow were the same as those of the steady theory. The scale of the calculation is approaching to the scale that was observed by the VLBI technique, which provides the current highest resolution for YSO jets.     

The two-dimensional structure of thin accretion disks

The two-dimensional structure of a thin accretion disk in the vicinity of a Schwarzschild black hole after passing a marginally stable orbit (r< 3r _g is discussed in terms of the Grad-Shafranov hydrodynamic equation. The accretion disk is shown to be sharply compressed as the sonic surface is approached, so the mass flow here is no longer radial. As a result, the dynamic forces ρ[(v ?)v] _θ , which are equal in magnitude to the pressure gradient ? _θ P on the sonic surface, become significant in vertical balance. Therefore, the disk thickness in the supersonic region (and, in particular, near the black-hole horizon) may be assumed to be determined not by the pressure gradient but by the shape of ballistic trajectories.     

Angular momentum transport in quasi-Keplerian accretion disks

We reexamine arguments advanced by Hayashi & Matsuda (2001), who claim that several simple, physically motivated derivations based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the nature of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expression for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The derivation is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.     

Magnetohydrodynamic instabilities and turbulence in accretion disks

In this paper we review the possibilities for magnetohydrodynamic processes to handle the angular momentum transport in accretion disks. Traditionally the angular momentum transport has been considered to be the result of turbulent viscosity in the disk, although the Keplerian flow in accretion disks is linearly stable towards hydrodynamic perturbations. It is on the other hand linearly unstable to some magnetohydrodynamic (MHD) instabilities. The most important instabilities are the Parker and Balbus-Hawley instabilities that are related to the magnetic buoyancy and the shear flow, respectively. We discuss these instabilities not only in the traditional MHD framework, but also in the context of slender flux tubes, that reduce the complexity of the problem while keeping most of the stability properties of the complete problem. In the non-linear regime the instabilities produce turbulence. Recent numerical simulations describe the generation of magnetic fields by a dynamo in the resulting turbulent flow. Eventually such a dynamo may generate a global magnetic field in the disk. The relation of the MHD-turbulence to observations of accretion disks is still obscure. It is commonly believed that magnetic fields can be highly efficient in transporting the angular momentum, but emission lines, short-time scale variability and non-thermal radiation, which a stellar astronomer would take as signs of magnetic variability, are more commonly observed during periods of low accretion rates. Received October 12, 1995 / Accepted November 16, 1995     

Models of high-luminosity accretion disks surrounding black holes

The problem of steady-state accretion to nonrotating black holes is examined. Advection is included and generalized formulas for the radiation pressure in both the optically thick and thin cases are used. Special attention is devoted to models with a high accretion rate. Global solutions for accretion disks are studied which describe a continuous transition between an optically thick outer region and an optically thin inner region. It is shown that there is a maximum disk temperature for the model with a viscosity parameter α = 0.5. For the model with α = 0.1, no optically thin regions are found to exist for any accretion rate.     

Pulsational instability of accretion disks around compact objects

Linear analysis shows that radial oscillations in accretion disks around compact object are overstable to axisymmetric perturbation under a variety of conditions. Furthermore, numerical simulations confirm that the radial oscillations induce quasi-periodic modulations of the disk luminosity. The disk oscillation model may be responsible for quasi-periodic oscillations (QPOs) observed in low mass X-ray binaries (LMXBs), cataclysmic variables (CVs), and other compact objects.     

Non-LTE models of the accretion disks of UX Ori stars

The results of a non-LTE analysis of a number of spectral lines formed in the accreting envelopes of UX Ori stars are given. The accretion rate is estimated from an analysis of the first three lines of the Balmer series: M _a = 10^?8 ?10^?9 M _⊙ The gas temperature in this region is about 10,000 K. In the immediate vicinity of the star there is a hotter region, with T > 15,000 K, in which the 5876 Å line of neutral helium, observed in the spectra of these stars, is formed. The region of formation of this line has a small geometrical thickness, covers a small fraction of the star’s visible disk, and evidently consists of the site of contact of the accreting gas with the stellar surface. The low gas rotation rates in this region (150–200 km/sec) may mean that rapid rotation of the accreting gas is damped by the star’s magnetic field, which is strong enough to affect the gas stream. We estimate the magnetic field strength in this region to be about 150 G.     

A unified description of the global structure of accretion disks

The global structure of accretion disks is investigated in a unified scheme. We use a general radiative cooling formula applicable to both optically thick and thin regimes and we include radial advection cooling. Within the -viscosity models, we found distinct families of global solutions. If the accretion rate is low, there are three non-intersecting solutions, corresponding to optically thick and thin, local cooling and optically thin advection cooling. If the accretion rate is high, and the viscosity coefficient is large, the two local cooling solutions coincide at radii R₁ and R₂ and exist independently below R₁ and above R₂ while the advection solution is stable at all radii. If the accretion is high and the viscosity is low, the two optically thin solutions will cross each other while the optically thick solution exists at all radii.