Abstract

In this lecture the basic theory of accretion disks is reviewed, with emphasis on aspects relevant for X-ray binaries and cataclysmic variables. The text gives a general introduction as well as a selective discussion of a number of more recent topics.

1.1 Introduction

Accretion disks are inferred to exist as objects of very different scales: millions of kilometers in low mass X-ray binaries (LMXB) and cataclysmic variables (CV), solar-radius-to-AU-scale disks in protostellar objects, and AU-to-parsec-scale disks in active galactic nuclei (AGN).

An interesting observational connection exists between accretion disks and jets (such as the spectacular jets from AGN and protostars) and outflows (the “CO-outflows” from protostars and the “broad-line regions” in AGN). Lacking direct (i.e., spatially resolved) observations of disks, theory has tried to provide models, with varying degrees of success. Uncertainty still exists with respect to some basic questions. In this situation, progress made by observations or modeling of a particular class of objects has direct impact on the understanding of other objects, including the enigmatic connection with jets.

In this lecture I concentrate on the more basic aspects of accretion disks, but an attempt is made to mention topics of current interest as well. Some emphasis is on those aspects of accretion disk theory that connect to the observations of LMXB and CVs. For other reviews on the basics of accretion disks, see Pringle (1981) and Papaloizou and Lin (1995). For a more extensive introduction, see the textbook by Frank et al. (2002). For a comprehensive text on CVs, see Warner (1995).

Abstract

X-ray binaries are responsible for the bulk of the X-ray emission of our own galaxy. A lot has been learned about these bright X-ray sources since the beginning of X-ray astronomy, but significant questions are still open. These questions are related to the origin and evolution of these sources, and to how their properties depend on those of the parent stellar population. The discovery of several populations of X-ray binaries in external galaxies with Chandra, and to a lesser extent with XMM-Newton, gives us tools to look at these sources in a new way. Not only can we reconsider long-standing questions of galactic studies, such as the origin of low-mass X-ray binaries, but also we can look at the entire gamut of X-ray binary properties in a range of environments, from actively star-forming galaxies to older stellar systems. These observations have led to the discovery of several ultraluminous X-ray sources, thereby introducing new interesting possibilities for our understanding of X-ray binaries and possibly opening new paths to the discovery of the elusive intermediate-mass black holes.

5.1 Introduction and chapter outline

X-ray astronomy began with the unexpected discovery of a very luminous source, Sco X-1 (Giacconi et al., 1962), the first galactic X-ray binary (XRB) ever to be observed. XRBs, the most common luminous X-ray sources in the Milky Way, are binary systems composed of an evolved stellar remnant (neutron star [NS], black hole [BH], or white dwarf [WD]), and a stellar companion (for reviews on XRBs, see Lewin et al., 1995; Lewin and van der Klis, 2006).

8.1 Introduction

Astronomers have a remarkably successful theory for stars and stellar evolution. This success is due in part to the simplicity of spherical symmetry and steady-state equilibrium. Stars can be modeled using a series of time-independent equations that depend on only one spatial coordinate, namely the radius of the star. But the universe is a much more dynamic and active place than is implied by the stars alone. Some of the most energetic photons that astronomers observe originate not within stars but in orbiting disks of gas. This realization has brought the study of accretion disks to the forefront of high-energy astrophysics.

The idea of an orbiting disk of gas in a context other than that of a nascent solar system or spiral galaxy can be traced at least as far back as the work of astronomer Gerard Kuiper on mass transfer in close binary stellar systems. He noted that in such systems, gas can flow through a stream from one star to the other. Kuiper realized that the gas would possess sufficient angular momentum that it must go into orbit around the attracting star, forming a ring.

In 1955, John Crawford and Robert Kraft published a paper (Crawford and Kraft, 1956) that proposed an orbiting ring model for AE Aquarii, a short-period binary star system that showed significant episodic variability. The masses of the stars and the sizes of their orbits were such that mass transfer from one star to the other was likely.

3.1 Accretion from the ISM and winds

3.1.1 Introductory remarks on white dwarfs

The spectra of white dwarfs (WD) are classified according to the scheme devised by Sion et al. (1983), of which we need here to use only the types DA (with strong H lines), DB (with He I lines and no H), and DZ (metallic lines, e.g., Ca, but excluding C, subdivided into DAZ and DBZ). In addition, magnetic fields in WDs play important roles in accretion processes. Their occurrence in isolated form (or as members of noninteracting binaries) is observed by Zeeman splitting or polarization, and the distribution of field strengths appears bimodal: Wickramasinghe and Ferrario (2000, 2005) conclude that ~16% of WDs have strong fields (≥0.5 MG); a much smaller fraction have lower fields, but there are indications of a rise of up to 25% at the kG level.

3.1.2 Accretion from the ISM

Most isolated WDs are of type DA or DB, but a small fraction at the cool end of the WD sequence are of type DZ (Fig. 3.1). The reason for ignoring carbon in this spectral type is because it can be dredged up from the interior, whereas the other metals must have a different origin. Levitation by radiation pressure is not strong enough to keep metals in the atmospheres of such stars (for T < 40,000 K), and gravitational settling time scales are short compared with the cooling time scale, so the metals must have been delivered from outside the star – such as from the interstellar medium (ISM).

Abstract

These notes resulted from a series of lectures at the IAC winter school. They are designed to help students, especially those just starting in subject, to get hold of the fundamental tools used to study accretion powered sources. As such, the references give a place to start reading, rather than representing a complete survey of work done in the field.

I outline Compton scattering and blackbody radiation as the two predominant radiation mechanisms for accreting black holes, producing the hard X-ray tail and disk spectral components, respectively. The interaction of this radiation with matter can result in photoelectric absorption and/or reflection. While the basic processes can be found in any textbook, here I focus on how these can be used as a toolkit to interpret the spectra and variability of black-hole binaries (hereafter BHB) and active galactic nuclei (AGN). I also discuss how to use these to physically interpret real data using the publicly available XSPEC spectral fitting package (Arnaud, 1996), and how this has led to current models (and controversies) of the accretion flow in both BHB and AGN.

6.1 Fundamentals of accretion flows: observation and theory

6.1.1 Plotting spectra

Spectra can often be (roughly) represented as a power law. This can be written as a differential photon number density (photons per second per square cm per energy band) as N (E) = N 0E-r, where Γ is photon index. The energy flux is then simply F (E) = EN(E) = N0E-(r-1) = N0E-α, where α = Γ − 1 is energy index.