Accretion Disks in Compact Stellar Systems

The following sections are included:

• PREFACE

• CONTENTS

Books about accretion discs have been written before but none cover the subject in as much detail as is done here. This book takes the reader from the basic concepts of accretion discs to the most up-to-date work currently being carried out in the subject. Both observational and theoretical work are covered. As such, it is suitable as an introduction for those who have never worked in this area with many useful references to other reviews. It will also be extremely useful for astronomers who are working in the area. Each chapter concentrates on a different aspect of accretion discs and is written by an expert in each field. The reader is given many references with which they could pursue further any aspect of interest…

We review the accretion disk limit cycle mechanism within the context of dwarf nova accretion disks. We begin with a discussion of the basic physics behind the instability, and then proceed to an overview of the comparison between theory and observation.

We consider the transport of angular momentum in accretion disks that are low mass, in the sense that the gravitational forces produced by the material in these disks has a negligible effect on disk dynamics. There is no established consensus on how this transport takes place. We note that for phenomenological reasons the traditional α model is probably not a good description of real disks. Here we briefly review a few of the more promising models. These include models in which angular momentum transport is driven by shocks, and by magnetic field instabilities. The latter is more promising, but requires a dynamo. We note that the direction of angular momentum transport due to convection in a conducting disk is not known as competing mechanisms are at work. We briefly discuss a number of possible dynamo mechanisms, and their problems. We then give a detailed exposition of the internal wave driven dynamo model, in which internal waves excited at large radii drive an α – Ω dynamo. The azimuthal magnetic field produced in this way is unstable to a magnetic shearing instability (MSI), which drives nearly isotropic turbulent eddys with typical fluid velocities of ~ V_A, where V_A is the local Alfvén speed. The scale of the eddys is ~ V_A/Ω, where Ω is the local rotational frequency. This turbulence leads to a saturation of the dynamo when V_A ~ (H/r)^2/3c_s, where H is the half-thickness of the disk, c_s is the local sound speed, and r is the radial coordinate. This gives rise to an effective dimensionless viscosity coefficient ~ (H/r)^4/3 and vertical and radial diffusion coefficients which are ~ (H/r)^4/3Hc_s. The resulting vertical diffusion of entropy will have a substantial effect on detailed models of vertical structure in accretion disks. Viscous and thermal instabilities of very hot disks, those dominated by radiation pressure and electron scattering, are substantially moderated in this model. We note that the MSI largely suppresses the Parker instability in accretion disks.

This review discusses the emission lines from accretion disks in dwarf novae, anti-dwarf novae, UX UMa stars, and to a lesser extent classical novae. Doppler tomography and direct fits to time-averaged line profiles show that the distribution of the hydrogen line emission is peaked towards the center of the disk and can be approximated by a radial distribution f(R) α R^-β with β ~ 1.5 – 2.0. The Balmer decrement flattens somewhat towards the center. The equivalent widths of the emission lines are strongly correlated with the absolute visual magnitudes of the systems, becoming weaker as the systems become brighter. They are also correlated with orbital inclination, becoming greater as the orbital inclination increases. Models invoking optically-thin viscous accretion disks fail to reproduce the observed line strengths and their correlation with orbital inclination, and they are unable to account for the detailed behavior of the emission lines during the outbursts of dwarf novae. There is evidence that the emission lines are produced by irradiation of the disk by the boundary layer and the central star. Models invoking irradiation agree qualitatively with the observational data.

Observations of eclipse light curves are being used to make maps of the accretion disks in eclipsing cataclysmic variables. By observing the structure of real accretion disks, we can test models of the structure of accretion disk atmospheres, learn about accretion disk viscosity by tracking time-variations in the structure of dwarf nova disks undergoing outbursts, and measure mass accretion rates in systems at different binary periods to test ideas about the long-term evolution of cataclysmic variables. The observed disk structures in dwarf novae during outburst and in long-period nova-like variables confirm the T α R^-3/4 law predicted by theory for steady-state disks. However, much flatter radial profiles are found in the disks of quiescent dwarf novae, which appear to be optically-thin, and in nova-like variables with periods between 3 and 4 hours, which may be driving an accretion disk wind. This chapter reviews eclipse mapping methods, and discusses some of the challenges that accretion disk maps are presenting to accretion disk theory.

The construction of self consistent model atmospheres for stationary accretion discs is reviewed and a method to calculate the corresponding hydrostatic structure and the radiation field is described. The importance of such models for the understanding of the continuum and the line emission as well as the necessity to construct the model in a self consistent way are discussed and demonstrated.

We discuss close binary accretion flows onto magnetic neutron stars and white dwarfs. The original picture of disc flow disrupted by flow along magnetic fieldlines at some inner radius fails in most cases, the exception being neutron–star binaries in which the companion overflows it Roche lobe. In wind accretion, or Roche lobe overflow onto a magnetic white dwarf, the flow is ill–understood, and disc models cannot account for the observed angular momentum budget of the binary. We discuss recent progress in these areas.

In this chapter, the focus is on two phenomena associated with non-magnetic cataclysmic variables in the high state, namely the optically-thick boundary layer and high velocity outflow. The state of the art in hydrodynamic modelling of the boundary layer is described and is shown to support the 'classical' concept of the boundary layer as an optically-thick, physically-thin equatorial belt around the accreting white dwarf. The observed and derived properties of cataclysmic variable winds are summarised. A critical overview of the possible physical relation between mass loss and the boundary layer is then given. Particular attention is paid to what has been and can be learned from X-ray and higher sensitivity ultraviolet observations.

The problem of the interaction between the stream of gas from the inner Lagrangian point and the accretion disk is reviewed. Observations of both cataclysmic variables and low mass x-ray binaries indicate that in addition to phenomena associated directly with the bright spot at the point of impact, the accretion disk exhibits regions of vertical thickenings at a few binary phases. It is pointed out that such a vertical structure is indeed expected on the basis of theoretical considerations. It is argued that a combination of future observations and calculations can yield important system parameters and a better understanding of the processes associated with accretion onto compact objects.

Tidal effects exerted by the secondary star on accretion disks are discussed in compact close binary systems. Non-axisymmetric structures of accretion disks are studied by the following three methods: (1) simple periodic particle orbits in the binary potential, (2) the perturbation method, and (3) full hydrodynamic simulations. Disk radius variations in the outburst cycle of dwarf novae are then studied. They manifest themselves most clearly in the tidal effects or accretion disks. The tidally driven eccentric instability (or the tidal instability) in an accretion disk is then discussed in connection with the superhump phenomenon in SU UMa-type dwarf novae. It is argued that the superoutburst phenomenon in SU UMa stars is most likely explained by an interplay of two kinds of intrinsic instabilities within a disk: the thermal instability and the tidal instability.

X-ray observations of low-mass X-ray binaries and black hole candidates are reviewed. In these sources, an accretion disk is considered to extend close to a neutron star or a black hole and to mainly govern the appearance of the X-ray emission. These sources generally show soft and hard spectral states, which possibly correspond to two interchangeable states of the inner part of the accretion disk. On the other hand, the outer part of the disk reflects, reprocesses and sometimes obscures X-rays from the central compact star. Similarities and differences between neutron star and black hole candidate sources are also discussed.

We review the properties of the X-ray illumination model for soft X-ray transients and its evolutionary consequences.

The understanding of stellar accretion disks has made revolutionary progress since the standard, steady disk model was proposed by Shakura and Sunyaev in 1973 Observations of the accretion disks in close binaries, especially in black-hole candidates, however, have revealed rather complex behavior, which cannot simply be described by steady-state picture of the standard model. We thus need to modify the standard model so as to explain a variety of observations. We first discuss the thermal limit-cycle instability caused by partial ionization of hydrogen (and helium), which seems to be responsible for X-ray nova eruptions. Next, we consider the role of high-energy phenomena, such as Compton scattering and e⁺e^-pair production, in the structure and the stability of hot accretion disks. Finally we summarize other kinds of disk instabilities, such as the tidal instability, irradiation-induced instabilities, and radial and nonradial disk pulsation.