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.