Introduction to the conference
Global time-dependent simulations provide a means to investigate time-dependent dynamic evolution in accretion disks. I will discuss ongoing efforts to extend previous local simulations through a systematic effort to develop fully global three-dimensional simulations. In initial work the nonlinear development of the magnetorotational instability is investigated using a time-explicit finite difference code written in cylindrical coordinates. Both a Newtonian, and a pseudo-Newtonian potential have been used. Two simplifications are also explored: a cylindrical gravitational potential (the ``cylindrical disk''), and axisymmetry. The results from those simulations are compared with fully three dimensional global simulations.
Turbulence, Convection and Angular Momentum Transport in Protoplanetary Accretion Disks.
We study the three-dimensional global structure of the flow-pattern in a convectively unstable non magnetic protoplanetary accretion disk. For that purpose, we use a versatile 3D hydro-code TRAMP, which incorporates radiation transport. With this code we are able, for the first time, to simulate the large-scale structure of circumstellar disks as they develop under the assumption of an artificial heating but without an explicit alpha viscosity. We find that axisymmetry is broken and density fluctuations with an azimuthal wavelength of several pressure scale heights form. Preliminary results show that the efficient turbulent transport of angular momentum is bound outward and corresponds to a turbulent alpha of the order of 10-4-10-3. The previous contradictory results by Stone and Balbus 1996, which indicated inward transport of angular momentum, can be explained by the specifics of their simulation.
Global disk simulations of magnetic shear-flow instability
In order to explain the high eddy viscosity needed for the time-scale of star formation, we perform global simulations of accretion disks based on the ZEUS-3D code. The magnetic shear-flow instability (Balbus-Hawley instability) turns out to be a suitable and quick mechanism to excite turbulence. We measure the viscosity parameter alpha_SS from the numerical results. Indications for dynamo action are found in the turbulent disk; we report about the correlation between helicity of the flow and the equivalent alpha-effect of the dynamo action.
Accretion-Ejection Instability in Black-hole binaries
We have recently found that a magnetized disk could be unstable to a large-scale instability. It has the form of a spiral density wave in the innermost region of the disk, extracting energy and angular momentum and transfering them to a Rossby vortex. They will end up propagated by Alfven waves to the disk corona, where they can power a jet or a wind.
I will show how this instability might explain the low-frequency Quasi Periodic Oscillation (QPO) observed in black-hole binaries. Its frequency and certain observed properties of the QPO in various sources favour this identification. Then I will show how, in the special case of the micro-quasr GRS 1915+105, this leads us to a "magnetic flood" scenario, where the 30 mn cycles of the source (leading to supra-luminal ejections) would be controlled by the magnetic flux in the disk.
On the Blandford-Znajek energy loss of the rotating black hole.
V.S.Beskin, I.V.Kuznetsova P.N.Lebedev Physical Institute, Moscow, RUSSIA Moscow Institute of Physics and Technology The Grad-Shafranov approach to the problem of the structure of the black hole magnetosphere is discussed. It is demonstrated that the number of the boundary conditions is enough to determine not only the longitudinal electric current, but also the angular velocity of a flow as a solution of a problem. As a result, the energy loss is determinate by the physical parameters at the particle creation region, not by the "boundary conditions" at the event horizon.
The Dynamic Corona of Accretion Disks
The spectral properties of accretion disk coronae are reviewed. The improved quality of new X-ray observations resulted in strong constraints on the corona models. The X-ray data suggest that the coronal plasma is ejected with mildly relativistic velocity. The phenomenon of plasma ejection affects the observed radiation at all wavelengths. In particular, the presence of coronal outflows in quasars can explain their mysterious UV-polarization.
The problem of launching an accretion disc wind along a dynamo-generated magnetic field is considered. When the diffusive nature of the disc and the curvature of the field are taken into account, the standard critical launching angle of 60 degrees is lowered to nearer 20 degrees. Only photospheric temperatures are needed to generate a significant wind mass flux.
Magnetized Accretion Disks in Classical T Tauri Systems
We study the impact of a dipolar magnetic field rooted in the central star of a classical T Tauri system on the protoplanetary disk by solving the equations of motion, mass conservation, gas energy conservation, radiative energy conservation, and the induction equation. The computations are time dependent and assume axisymmetry. While there is little impact on the disk for weak fields, stronger fields can change the structure of the disk and finally completely disrupt its inner parts. Magnetic coupling between the central star and the disk is crucial for the star's anglular momentum evolution. The total magnetic field consists of the stellar magnetic field and a component generated by rotational shear inside the disk and between disk and star.
Modelling the X-ray/gamma-ray spectra and temporal varibility of accreting compact objects
Accreting black holes and neutron stars in their hard (low) state show not only very similar X/gamma-ray spectra but also the behaviour of their light curves is quite similar which can be quantified in similar power-density spectra and Fourier-frequency-dependent time/phase lags. Taken together this argues for the common mechanism of the X/gamma-ray production in these objects. This mechanism is probably a property of the accretion disk only since it is little affected by the presence of the neutron star surface. I review the X/gamma-ray properties of accreting compact objects paying most attention to the spectral variability and Fourier-frequency-dependent time/phase lags. I also discuss the models which explain the basic observational facts.
I also present a model which describes well the time-averaged energy spectra as well as temporal characteristics such as power density spectrum, time/phase lags, and coherence function of accreting compact objects. In this model, X/gamma-rays are produced in compact magnetic flares at radii < 100GM/c^2 from the central object. The tendency for magnetic loops to inflate and detach from the underlying accretion disk causes the spectrum of a flare to evolve from soft to hard due to the decrease of the feedback from the cold disk, so causing time delays between hard and soft photons. The observed time lags are identified with the evolution time scales of the flares, which are of the order of the Keplerian time scale.
Studies of geometry of the accretion flow through X-ray reprocessing
The interaction of hard X-ray radiation with optically thick phase of accreting plasma produces spectral features that can be used to constrain the geometry and physical conditions of the accretion flow. The X-ray reprocessed component consists of the Compton reflected continuum with iron fluorescence/recombination line and absorption edge imprinted on it. The amplitude of this component gives the solid angle of the reprocessor from the hard X-ray source, the temperature and ionization state of the reprocessor can be inferred from the energy and strength of the iron spectral features, while the plasma's velocity will be revealed by broadening and smearing of the spectral features.
I am going to report on recent studies of accretion flow physical conditions and geometry using the above techniques in a variety of accreting systems: Galactic transient and persistent X-ray sources, active galactic nuclei. Properties of reprocessing in the low/hard state of both SXT and persistent sources and Seyfert 1 galaxies suggest similar geometry of accretion in those objects, probably involving a truncated optically thick disk. In their high/soft states GBH seem to have larger amplitude of reflection, strongly ionized and relativistically smeared, indicating a change of geometry in the high-low transition. In the Very High State of SXT, the reprocessed component is also present, it is very strongly ionized and relativistically smeared. The Narrow Line Seyfert 1 galaxies are proposed to be AGN's in the High (or Very High) State, and they show ionized reprocessing as well.
Accretion disc variability, in particular Dwarf-Nova and X-ray Nova outbursts, may, in principle, provide crucial informations about discs structure and physical mechanisms driving accretion. I will review the thermal-viscous disc instability model taking into account the most recent results. I will try to establish under what conditions this model may be considered as the model of disc outbursts in binary systems.
A Quasilinear Prediction for Helicity and Dynamo Activity in Accretion Disks
Magnetic field generation in ionized accretion disks is a critical part of the conversion of gravitational energy into local heating. Here we argue that this process can be understood in terms of mean-field dynamo theory, and derive the mean fluid helicity tensor from the theory of magnetic field instabilities in accretion disks. In particular, we show that treating buoyancy as a weak correction to the rotationally driven (Velikhov-Chandrasekhar or Balbus-Hawley) instability produces a helicity tensor which is consistent with the results of disk simulations with vertical structure, and inconsistent with previous treatments based on the Parker instability. The resulting dynamo competes with incoherent dynamo effects in the simulations, and in slim disks, but dominates in very thin disks. The effective value of alpha is expected to depend somewhat on the details of vertical structure and the disk geometry, and does not decrease dramatically in very thin disks. The coherent dynamo produces a vertical magnetic field profile, and consequently a vertical heating profile, that peaks at least a density scale height away from the disk midplane. When the background structure of the disk is ignored, the turbulence will still generate significant helicity, and drive a mean-field dynamo, through a spontaneous symmetry breaking mechanism. This last effect accounts for the efficient generation of large scale magnetic fields in simulations without vertical disk structure.
1. N.I.Shakura. Complex Accretion Disk Behaviour in Her X-1
The observed properties of X-ray dips in the light curve of Her X-1 allow to reconstruct many features of a twisted tilted accretion disk around the neutron star in Her X-1. The dips are produced by eclipse of the central source by gaseous streams that come from the vicinity of the inner Lagrangian point of the optical star nearly filling its Roche lobe. The shadows produced by the accretion disk make the illumination of the optical star atmosphere variable and asymmetric so the gas flows are non-complanar with the orbital plane. The disk precession is controlled by both the dynamical action of the streams and tidal forces. The long-term stability of the 35-day cycle is likely to be associated with triaxial free precession of the neutron star, which is evidenced by the X-ray pulse profile changes.
2. Shakura N.I., Lipunova G.V., Sunyaev R.A., Time-dependent accretion disks in binaries. The analytic investigation of time-dependent accretion in disk is carried out. We consider a time-dependent disk in a binary system at outburst which has a fixed tidally truncated outer radius. The fully analytic solutions for Keplerian disks are obtained characterized by power--law variations of accretion rate with time. The solutions supply asymptotic description of the disk evolution in flaring sources in the periods after outbursts. While the bolometric luminosity varies as a power law, the X-ray flux of multicolor (black-body) disk is obtained to vary quasi-exponentially. The application to X-ray novae is briefly discussed. The case of time-dependent advective disk when the exponential variations of accretion rate can occur is discussed.
G. Ruediger & U. Ziegler
The relation between angular momentum transport and dynamo alpha in stratified, weakly magnetized accretion disks
I want to discusse the problem of formation of disc in Binaries. It is shown that the theory of "hot spot" is wrong. The causes of sparial shock wave are examined. 3D calculation show the structure of streams in Binaries.
Physics of advective magnetized accretion discs.
Advection is important for accretion disc structure at large luminosities, approaching the critical Eddington. For optically thin branch of the accretion disc solution magnetic field play an important role for heating, cooling, and momentum transfer properties. Solutions of the accterion disc structure are discussed where importance of magnetic field is critical.
Turbulence in warped accretion discs
Warps are seemingly common in accretion discs, and have been utilized to explain phenomena as diverse as the 35-day cycle in Her X-1 and the water masers in active galactic nuclei, such as NGC 4258. On the local scale a warp in an accretion disc drives an epicyclic motion. We study the interaction of this epicyclic motion with the magnetohydrodynamic turbulence in the accretion disc. The turbulence damps the epicyclic on a time scale of 25 orbital periods. While the accretion is driven by the Maxwell stress, the epicyclic motion is damped by the Reynolds stress. An important mechanism in this process is a parametric instability, that couples the epicyclic motion to an inertial wave.
Two-dimensional rotating accretion flows around black holes.
We present results of two-dimensional time-dependent hydrodynamical simulations of non-radiative rotating accretion flows into black holes. Properties of the accretion flows strongly depend on the strength of viscosity. In the low-viscosity case the accretion flows are convectively unstable and no powerful outflows are formed. In the high-viscosity flows the small-scale convective instability is suppressed, and the accretion flows are accomanied by powerful outflows and/or global meridional circulations of matter depending on both the strength of viscosity and the adiabatic index.
The interaction of an accretion disc with a central compact object results in torques which may cause alignment of angular momentum vectors and spin up or spin down. Some recent results on these processes are discussed.
In massive X-ray binaries, the captured wind matter falls onto the compact object with modest angular momentum, j, just about critical for formation of an accretion disc. We acess the mechanism of accretion in this situation, which is neither spherical nor it can be described by the standard accretion disc model. We assume that the matter infall is symmetric with respect to the plane of the binary system. A small-scale accretion disc forms in this plane, being fed at each radius by the wind matter. In contrast to the standard accretion disc, here viscous stresses are negligible: the disc accretes fast owing to its relatively low angular momentum and also owing to the radial pushing inwards by the infalling matter. The disc is essentially non-Keplerian and it has a highly super-sonic radial velocity. If the compact object is a black hole, which does not possess any hard surface, then the disc is the main source of X-rays. Absorption of the infalling matter by the disc is accompanied by a strong shock which transforms the infall kinetic energy into heat and radiates it away. Most of the energy is emitted in hard X-rays with a standard Comptonized spectrum, a power-law with photon index Gamma ~ 1.5 - 2 and a break at ~ 100 keV.
Discoseismology - may reveal the disc structure in short period cataclysmic variables.
For some cataclysmic variables with small mass ratios, the accretion disc may expand beyond the tidal radius, and the tidal interaction between the secondary and particles in the disc may create a spiral or ring pattern in the disc. This pattern may be detected as modulation in the light curve.
We have observed a group of short period, low mass ratio, cataclysmic variables - the AM CVn stars, or helium cataclysmics, - where the the light curves are modulated by a series of harmonic frequencies, which we interpret as a sign of symmetrical structures in the disc. Some of these objects behave like the nova- like variables, and cycle from a low to a high state, and the harmonic pattern changes. This may indicate dynamic changes in the disc, going from a large optical thick state to a smaller optical thin state.
In one or two cases we also detect the superhump period in the fine structure of the harmonic frequency pattern.
Global MHD Simulations of Disks and Jet Formation
We present the results of three-dimensional global magnetohydrodynamic simulations of differentially rotating disks. An equilibrium model of magnetized torus is adopted as an initial condition. When the torus is threaded by large-scale open magnetic fields, magnetically driven bipolar jet emanates from the torus. The jet is hollow shaped and gradually collimated along the rotation axis. Owing to the growth of non-axisymmetric instabilities, the jet also shows helical structure. Since the jet extracts angular momentum from the torus, it enhances accretion of the disk material.
When a disk is threaded by the dipole magnetic field of the central object (e.g., a protostar or a neutron star), twist injection from the disk drives expansion of magnetic loops connecting the central star and the disk and creates current sheets inside the expanding loops. Magnetic reconnection taking place in the current sheet heats up the plasma and generates X-ray emitting outflows. The magnetic interaction between a central star and the disk inevitably accompanies X-ray flares and outflows.
Finally, we would like to show the results of 3D global MHD simulations of the Parker-shearing instability in a torus initially threaded by toroidal magnetic fields. When the initial magnetic field is nearly equipartition strength (plasma beta=1), magnetic flux buoyantly escapes from the disk and creates loop-like structures similar to those in the solar corona. The magnetic loops are twisted by the differential rotation of the disk. We can observe expansion of the loops and associated outflows.
Pressure-driven outflow and magneto-centrifugal wind from a dynamo active disc.
We present a numerical model of an accretion disc with mean-field dynamo action that develops pressure-driven collimated outflow near the rotation axis and a centrifugally driven uncollimated wind in the outer parts. The jet is collimated and confined by the azimuthal magnetic field that is produced by the dynamo in the disc and advected to the disc corona. The jet is hot and dense, but has low angular momentum.
Long-term evolution of a dipolar-type magnetosphere interaction with an accretion disk
MHD simulations will be presented of a stellar dipolar-type magnetic field which is anchored both in the central star and the surrounding Keplerian disk. The simulations lasts for several hundreds of Keplerian periods of the inner disk radius und shows indication for a possibly stationary final state. The dipolar field disappears and a monopole-type field develops enclosing a neutral field line.
Observations of outflows from the peculiar symbiotic star CH Cygni
Behaviour of CH Cyg does not fit classical models of symbiotic stars. Its symbiotic activity is intermittent and revealed itself first in 1963 only. The most spectacular active period up to now was observed in 1977-1987, and it was culminating with the ejection of massive, collimated bipolar jets, detected in radio and optical wavelengths. In 1990s another type of high-velocity outflows (possibly centrifugal, velocities up to 1000-2000 km/s) has been observed. A preliminary interpretation of the observed phenomena will be given in the framework of the magnetic rotator model.
I will present steady-state calculations of self-similar magnetized accretion discs driving cold, adiabatic, non-relativistic jets. For the first time, both the magnetic torque due to the jets and a turbulent "viscous" torque are taken into account. This latter torque allows a dissipation of the accretion power as radiation at the disc surfaces, while the former predominantly provides jets with power. The parameter space of these structures has been explored. It is characterized by four free parameters, namely the disc aspect ratio and three MHD turbulence parameters, related to the anomalous magnetic diffusivities and viscosity. It turns out that launching cold jets from thin, dissipative discs implies an anisotropic MHD turbulence. Jets that asymptotically reach a high Alfvenic Mach number are only produced by non-dissipative discs.
Origin of Astrophysical Jets
Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are extra galactic radio jets of active galaxies and quasars, jets from compact stars in binary systems, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the B field by the rotation of the disk drives the jets by magnetically and centrifugally driving matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now demonstrate the formation - acceleration and some collimation - of hydromagnetic jets from accretion disks. More recently, new theoretical and computational work at Los Alamos National Lab have shown the existence of collimated Poynting-flux outflows from accretion disks. The Poynting-flux outflows are of particular importance for the jets observed to emanate from active galactic nuclei and quasars.
The Magnetized Accretion-Ejection Flows and their Transport Issues.
I intend to present the recent developments of the investigations of the Grenoble group on the interaction of the magnetized accretion disk and its jets for both Young Stellar Objects and Active Galactic Nuclei. These flows have been studied for a wide interval of magnetic Reynolds numbers allowing regimes where both the magnetic torque and the turbulent viscous torque are relevant and regimes where the magnetic torque is dominant. To get such quasi stationary flows a rather large effective magnetic diffusivity is badly needed. The transport coefficient issue in these flows has to be solved under the conditions of jet launching, which requires a rough equipartition of magnetic and plasma pressures. As is well known, the magneto-shear instability is quenched in a magnetic field above equipartition. Another source of MHD instability is proposed to maintain the turbulence in this configuration, in both the disk and its jets.
The results of recent fully general relativistic 2D simulations of MHD flows in vicinity of a rotating black will be presented.
Jets in Protostellar Systems: Links to Star and Planet Formation
Molecular outflows and jets are one of the earliest manifestations of star formation. They are the strongest within the so-called, class 0 objects which are arguably the best existing candidates for protostars - systems that are less than a free-fall time old. Jet activity becomes less apparent as accretion disks around young stellar objects age and are used up and/or dispersed. There is also good evidence that jets are coupled to underlying, protostellar accretion disks.
I shall review some of the basic observations and current theoretical models for the origin, collimation, and role of accretion-driven, protostellar jets in the process of star and planet formation. I shall then focus upon the results of related numerical simulations on the origin of jets in the protostellar context. Finally, I shall discuss the results of some current simulations that we are performing on the acceleration of axisymmetric jets from accretion disks that are threaded by a wide variety of different initial magnetic configurations (eg. Blandford and Payne 1982, Pelletier and Pudritz, 1992).
We discuss the possibility of in situ generation of magnetic fields by plasma motion in collimated jet outflows. The flow is prone to dynamo action provided its streamlines are helical, i.e., both axial and azimuthal velocity components are of comparable magnitude. The resulting field configuration is nonaxisymmetric and represents a dynamo wave propagating along the jet axis.
Instabilities and collimation in magnetic jets.
Collimation of magnetically accelerated jets is conventially described in terms of axisymmetric models. Though these models include various possible instabilities, they do not include the one that is most destructive to the collimation process, namely kinking, which is intrinsically three-dimensional. In the absence of detailed simulations applicable to the astrophysical jet case, I speculate on what the consequences of this instability could be. The conclusions are that magnetically accelerated outflows probably need an external agent to provide high degrees of collimation, and propose that this agent is the poloidal magnetic field anchored in the disk. In highly collimated outflows kink instability destroys the azimuthal magnetic field effectively, so that a ballistic outflow with a low ratio of magnetic to kinetic energy flux results.
The development of both theory and numerical simulations of magnetically driven jets from accretion disks is reviewed, based on our recent works. Topics we discuss here are; why the terminal speed of the magnetically driven jet is nearly a Keplerian speed of the disk; how the muss flux of the magnetically driven jet is determined; and what is the the difference between the magneto-centrifugally driven jet and the magnetic pressure driven jet. New results about recent nonsteady numerical simulations of MHD jets are also included.
3-D Simulations of Astrophysical Jets: Stability Issues
We perform 3-D simulations of mass outflows emanating form the surface of a Keplerian accretion disk. The initial set up is an extension to 3-D of that used in our previous 2-D work (Ouyed and Pudritz, 1997, 1998, 1999). We focus on the issue of stability of 3-D jets; to try to elucidate the question of the plausible destruction of the toroidal component of the magnetic field by the kink instability in which case the standard picture of the toroidal collimation becomes questionable.
While some analytical and numerical work seem to indicate that jets ought to be unstable and must be collimated by other means then the toroidal picture (like the proposed poloidal collimation scenario), we demonstrate and discuss how non-linear effects allow the jet to settle into a stable configuration before the kink instability becomes important. We explain how mode coupling allows the more unstable region of the jet (the superAlfvenic region) to interact with the more stable subAlfvenic region in such a manner as to allow the jet to adjust (in its full length) into a more relaxed and stable configuration.
The "new" physics discovered/inherent in/to our simulated jets seem to indicate that astrophysical jets might after all be naturally stable.