Observations

Accretion on to stellar mass objects occurs in a wide variety of systems and yields a wide variety of observational behaviour. While there may be many arguments over detailed models, the broad basis of these differences is largely understood. Active galactic nuclei also come in many observed forms. From an observational viewpoint, they can be defined as apparently stellar sources but with non-thermal spectra, and, in cases where they can be determined, significant redshifts. Beyond this, we find a wide variety of properties, which we shall classify in more detail below. But in these cases it is not at all clear how these differences arise, or, indeed, whether one is even dealing with variants of a single basic model. We shall argue that the sources are all manifestations of accretion on to supermassive black holes (of order 108M⊙), although even this is still not universally accepted. Furthermore, for stellar-mass objects, at least in some cases, we have a complete picture of the system even if some of the details are missing. In no case do we have anything comparable for active galactic nuclei. That is not to say that there are no aspects of active galactic nuclei that are thought to be fairly well understood, but those that are do not include the mechanism of the basic energy source. Thus we have to try to extract from the available data what clues we can to the nature of the central engine.

In the decade since the second edition of this book, accretion has become a still more central theme of modern astrophysics. We now know for example that a γ-ray burst briefly emits a gravitationally powered luminosity rivalling the output of the rest of the Universe. This and other startling discoveries are a result of observational progress, driven as ever by technological advances. But these advances are also having a powerful effect on theory; modern supercomputers allow one to perform as a matter of routine calculations which were unthinkable a decade ago. This increasing capability will significantly alter the way theory is done, and indeed thought about.

The impact on accretion theory has already been profound. Most obviously, supercomputer simulations have been central in verifying that angular momentum transport in accretion discs is probably mediated by the magnetorotational instability. This opens the prospect of at last understanding how accretion is driven in the discs we see.

These changes and others make a new edition of this book timely. We are grateful for the opportunity of revising and extending the treatments of the earlier editions. As always, we have been obliged to be selective, but have tried to convey the essence of recent developments. In addition to discussing the new work on disc viscosity referred to above, we give a more thorough treatment of the thermal–viscous disc instability model now generally thought to be the basic cause of the outbursts of dwarf novae and soft X-ray transients.

Introduction

The importance of accretion as a power source was first widely recognized in the study of binary systems, especially X-ray binaries. This is still the area where the greatest progress in the understanding of accretion has been made. The reason for this is simply that, by their very nature, binaries reveal more about themselves, notably their masses and dimensions, than do other astronomical objects. This is particularly true in the case of eclipsing binaries, where we get direct information about spatial relations within the source. The importance of accretion is further manifested by the realization that probably a majority of all stars are members of binary systems which, at some stage of their evolution, undergo mass transfer.

The detailed study of interacting binary systems has revealed the importance of angular momentum in accretion. In many cases, the transferred material cannot land on the accreting star until it has rid itself of most of its angular momentum. This leads to the formation of accretion discs, which turn out to be efficient machines for extracting gravitational potential energy and converting it into radiation. This property has made accretion discs attractive candidates for the role of the central engine in quasars and active galactic nuclei; the question of whether or not this assignment is correct will be a theme of later chapters. In the context of binaries, however, their existence and importance is well attested through observation; we shall discuss the evidence for this in Chapter 5.

Astronomical spectropolarimetry is performed from the X-ray to the radio regimes of the electromagnetic spectrum. The following chapter deals with instruments and their components that are used in the wavelength range from 300 nm to 20 µm. After introducing the terminology and formalisms that are used in the context of astronomical spectropolarimeters, I discuss the most widely used optical components. These include crystal and sheet polarizers, fixed monochromatic and achromatic retarders, and variable retarders such as liquid crystals and photoelastic modulators. Since polarimetric measurements are often limited by systematic errors rather than statistical errors due to photon noise, I deal with these instrumentally induced errors in detail. Among these errors, I discuss instrumental polarization of various kinds and chromatic and angle of incidence errors of optical components. I close with a few examples of successful, modern night-time and solar spectropolarimeters.

Introduction

Scope of chapter

Astronomical polarimetry is performed over a large fraction of the electromagnetic spectrum, from X-rays to radio waves. The following chapter is restricted to the optical range that can be observed from the ground, i.e. 300 nm to 20 µm. Far-infrared polarimetry is described by Hildebrand in this volume. Information on polarimetry in the X-ray and radio regimes can be found in Tinbergen (1996).

Furthermore, the following text focuses on instruments for spectropolarimetry, i.e. the instrumental aspects of polarimetry with a spectral resolution that resolves spectral lines, either with a spectrograph or a filter, i.e. a spectral resolution of λ/δλ = R > 10,000.

The Sun is unique as an astrophysics laboratory because we can spatially resolve its structures in great detail and apply sophisticated diagnostic techniques that require high spectral resolution. The magnetic flux in the solar atmosphere occurs in extremely fragmented, nearly fractal form, with a range of spatial scales that extend well beyond the angular resolution limit of current telescopes and into the optically thin regime. The magnetic field leaves various kinds of “fingerprints” in the polarized spectrum. In the past only the fingerprints of the Zeeman effect have been used, but more recently new, highly sensitive imaging polarimeters have given us access to other physical effects. In particular a wealth of previously unknown spectral structures due to coherent scattering processes have been uncovered. These phenomena show up in linear polarization as a new kind of spectrum (the so-called “second solar spectrum”), which bear little resemblance to the ordinary intensity spectrum. Magnetic fields modify the coherent scattering processes and produce polarized spectral signatures that greatly extend the diagnostic range of the Zeeman effect. This diagnostic window has just been opened, and we are only now beginning to develop the needed diagnostic tools and apply them to learn about previously “invisible” aspects of solar magnetic fields.

The Sun's magnetic field — An introductory overview

Role of magnetic fields in astrophysics

Most of the matter in the universe, like stars, nebulae, and interstellar matter, consists of plasma, partially ionized gas with high electrical conductivity.

The main techniques used to diagnose magnetic fields in stars from polarimetric observations are presented. First, a summary of the physics of spectral line formation in the presence of a magnetic field is given. Departures from the simple case of linear Zeeman effect are briefly considered: partial Paschen-Back effect, contribution of hyperfine structure, and combined Stark and Zeeman effects. Important approximate solutions of the equation of transfer of polarized light in spectral lines are introduced. The procedure for disk-integration of emergent Stokes profiles, which is central to stellar magnetic field studies, is described, with special attention to the treatment of stellar rotation. This formalism is used to discuss the determination of the mean longitudinal magnetic field (through the photographic technique and through Balmer line photopolarimetry). This is done within the specific framework of Ap stars, which, with their unique large-scale organized magnetic fields, are an ideal laboratory for studies of stellar magnetism. Special attention is paid to those Ap stars whose magnetically split line components are resolved in high-dispersion Stokes I spectra, and to the determination of their mean magnetic field modulus. Various techniques of exploitation of the information contained in polarized spectral line profiles are reviewed: the moment technique (in particular, the determination of the crossover and of the mean quadratic field), Zeeman-Doppler imaging, and least-squares deconvolution. The prospects that these methods open for linear polarization studies are sketched.

Most observational work in astrophysics has so far been carried out mainly on the basis of the intensity of the radiation received from the object observed as a function of wavelength. However, an important and frequently overlooked aspect of electromagnetic radiation is its state of polarization, which is related to the orientation of the electric field of the wave. The state of polarization can be conveniently characterized in terms of four quantities that can be measured by furnishing our telescopes with a polarimeter. These observables are the four Stokes parameters (I, Q, U, V) which were formulated by Sir George Stokes in 1852 and introduced into astrophysics by the Nobel laureate Subrahmanyan Chandrasekhar in 1946. A quick, intuitive definition of the meaning of these four parameters can be obtained from Figure 1 of the chapter by Prof. Landi Degl'Innocenti in this book, which we borrowed for the poster announcing the Twelfth Canary Islands Winter School on Astrophysical Spectropolarimetry.

In physics laboratory experiments, where the magnetic field is known beforehand, the observed polarization signals are used to obtain information on the atomic and molecular structure of the system under study. In astrophysics we have the inverse problem, the magnetic field being the unknown quantity. To obtain information about cosmic magnetic fields, therefore, we have to learn how to interpret spectropolarimetric observations correctly by resorting to our knowledge of atomic and molecular physics.

This course is intended to give a description of the basic physical concepts which underlie the study and the interpretation of polarization phenomena. Apart from a brief historical introduction (Sect. 1), the course is organized in three parts. A first part (Sects. 2-6) covers the most relevant facts about the polarization phenomena that are typically encountered in laboratory applications and in everyday life. In Sect. 2, the modern description of polarization in terms of the Stokes parameters is recalled, whereas Sect. 3 is devoted to introduce the basic tools of laboratory polarimetry, such as the Jones calculus and the Mueller matrices. The polarization phenomena which are met in the reflection and refraction of a beam of radiation at the separation surface between two dielectrics, or between a dielectric and a metal, are recalled in Sect. 4. Finally, Sect. 5 gives an introduction to the phenomena of dichroism and of anomalous dispersion and Sect. 6 summarizes the polarization phenomena that are commonly encountered in everyday life. The second part of this course (Sects. 7-14) deals with the description, within the formalism of classical physics, of the spectro-polarimetric properties of the radiation emitted by accelerated charges. Such properties are derived by taking as starting point the Liénard and Wiechert equations that are recalled and discussed in Sect. 7 both in the general case and in the non-relativistic approximation. The results are developed to find the percentage polarization, the radiation diagram, the cross-section and the spectral characteristics of the radiation emitted in different phenomena particularly relevant from the astrophysical point of view.

Maser radiation occurs naturally in interstellar space. Masers provide extremely bright beacons that trace small scale structure in the host environments, which range from comets all the way to external galaxies. The radiation is sometimes, though not always, polarized. When it is, the polarization can reach much higher levels than in thermal sources—the radiation from some masers is fully polarized. This chapter provides an overview of astronomical masers, discusses the differences between maser and non-maser radiation and covers the fundamental theory of maser radiation and its polarization.

In 1963, the first radio emission from an interstellar molecule, the hydroxyl radical OH, was discovered. The ground state of this molecule produces four radio lines at wavelengths of approximately 18 cm (figure 1). When an atomic or molecular system radiates in several lines, certain ratios are expected between the line intensities. However, almost from the start the emission patterns displayed by the four OH lines in most astronomical sources were peculiar, deviating considerably from expectations. In 1965 these peculiarities culminated with the discovery of radio line emission with such exceptional properties, the emitting substance was dubbed ‘mysterium’ for lack of an obvious explanation. The exceptional properties included extremely high brightness, line widths much narrower than in all previous cases and substantial polarization. It did not take too long, though, to realize that ‘mysterium’ radiation was simply maser emission from interstellar OH.

Optical spectropolarimetry and broadband polarimetry in other wavebands has been a key to understanding many diverse aspects of AGN. In some cases polarization is due to synchrotron radiation, and in other cases it's due to scattering. Recognition of relativistically beamed optical synchrotron emission by polarization was vital for understanding blazars (BL Lacs and Optically Violently Variable quasars), both physically and geometrically. Radio polarimetry of quiescent AGN is equally important, again for both purposes. Scattering polarization was central to the Unified Model for Seyferts, Radio Galaxies and (high ionization) Ultraluminous Infrared Galaxies. It provides a periscope for viewing AGN from other directions. Finally, if we could understand its message, polarization would also provide major insights regarding the nature of the AGN “Featureless Continuum” and Broad (emission) Line Region.

I point out that high ionization ULIRGs have all the exact right properties to be called Quasar 2s. Mid-IR observations generally don't penetrate to the nucleus, greatly reducing their ability to diagnose the energy source. In particular, LINER ULIRGs aren't necessarily starburst-dominated, as has been claimed.

Seyfert Galaxies

Type 2 Seyferts

Polarization alignments and hidden Type 1 Seyfert nuclei

In the 1970s the continua of Seyfert 2s were decomposed into two parts: relatively red light from the old stellar population, and a bluer component modeled satisfactorily with a power law. The latter was called the “Featureless Continuum,” in a commendable attempt to avoid prejudice as to its nature.

After twelve years, the Canary Islands Winter School continues to provide a unique opportunity for the participants to broaden their knowledge in a key field of astrophysics. The idea works because promising young scientists and invited lecturers interact, learn and enjoy science in the pleasant environment of the Canary Islands.

The XII edition of the Canary Islands Winter School looked at the Universe from a relatively unexploited viewpoint, namely that fostered by a multidisciplinary branch of science which has a great future in store: spectropolarimetry. Thanks to theoretical and observational spectropolarimetry we will be able to explore new facets of the Universe while unveiling new discoveries still hidden in the electromagnetic radiation we receive. The large telescopes of the future – among them the 10.4 m Gran Telescopio Canarias – and advanced postfocus instrumentation should be designed with a view to rendering feasible high precision spectropolarimetric observations. The theoretical interpretation of observed polarization signals will allow new fundamental advances in our knowledge of cosmic magnetic fields. Spectropolarimetry could well be a revolutionary technique in the astrophysics of the XXI century. That is why the XII Canary Islands Winter School of Astrophysics has been devoted to this promising and exciting field.

The large-scale features of the magnetic field in the arms of the Galaxy have been traced by observations of polarized starlight, synchrotron emission, Zeeman splitting, and Faraday rotation. More recently, it has become possible to map fields in dense clouds by observations of polarized thermal emission from magnetically aligned dust grains. Observations at far-infrared and submillimeter wavelengths provide measurements of the field as projected on the sky at hundreds of points in individual clouds. In the polarization maps, especially when compared at several wavelengths, one finds examples of fields shaped by gravitational contraction, differential rotation, and compression. One also finds evidence for unresolved thermal structure and turbulence. To interpret the results one must understand the physical principles that relate emission, absorption, and scattering; and that relate polarization to the shapes and materials of the emitting dust grains. When these principles are applied to emission one finds that the degree of polarization in homogeneous clouds should be nearly independent of wavelength in the far-infrared and submillimeter portions of the spectrum. The steep polarization spectra actually observed can tentatively be understood if one assumes a heterogeneous temperature and radiation structure in which there is a correlation between temperature and grain alignment. The potential sources of systematic errors in polarization measurements are such that anyone entering the field must carefully review the appropriate observing and analysis techniques. With attention to the required techniques and with new instruments to be commissioned in the next few years it should become feasible to pursue scientific goals that have thus far been largely inaccessible.