Annu. Rev. Astron. Astrophys. 1997. 35: 445-502 Copyright © 1997 by Annual Reviews. All rights reserved

1. INTRODUCTION

Active Galactic Nuclei (AGN) produce enormous luminosities in extremely compact volumes. Large luminosity variations on time scales from years to hours are common. The combination of high luminosity and short variability time scale implies that the power of AGN is produced by phenomena more efficient in terms of energy release per unit mass than ordinary stellar processes (Fabian 1979). This basic argument leads to the hypothesis that massive black holes are present in the cores of AGN. Accretion of matter onto a black hole or extraction of its rotational energy can in fact yield high radiative efficiencies (Rees et al 1982, Rees 1984).

The basic AGN paradigm developed thus far consists of a central supermassive black hole, surrounded by an accretion disk, or more generally optically thick plasma, glowing brightly at ultraviolet (UV) and perhaps soft X-ray wavelengths. In the innermost region, hot optically thin plasma surrounding and/or mixed with the optically thick plasma gives rise to the medium and hard X-ray emission. Clouds of line-emitting gas move at high velocity around this complex core and are in turn surrounded by an obscuring torus or warped disk of gas and dust, with a sea of electrons permeating the volume within and above the torus.

In some systems, highly relativistic outflows of energetic particles along the poles of the rotating black hole, accretion disk, or torus form collimated radio-emitting jets that lead to extended radio sources. These AGN are called radio loud because their radio emission is comparatively strong; AGN without collimated jets, which therefore have weaker (but detectable) radio emission, are called radio quiet.

Variability studies have been essential in understanding the physics of the central regions of AGN, which in general cannot be resolved even with existing or planned optical/infrared (IR) interferometers. The time scales, the spectral changes, and the correlations and delays between variations in different continuum or line components provide crucial information on the nature and location of these components and on their interdependencies.

In recent years progress has been made on two fronts. First, for a handful of objects, large international collaborations have led to improved sampling, duration, and wavelength coverage in AGN monitoring campaigns. Second, the availability of uniform data sets like the International Ultraviolet Explorer (IUE) archive and various X-ray archives has made statistical comparisons possible among different classes of AGN and different wavelength bands. This article describes these recent advances, with particular emphasis on multiwavelength variability studies. Several excellent reviews have covered or touched on the subject in previous years in this series: intraday variability (Wagner & Witzel 1995), X-ray spectra and time variability of AGN (Mushotzky et al 1993), unified models for AGN (Antonucci 1993), and the earlier presentation of the black hole models for AGN (Rees 1984). We also note reviews elsewhere on AGN continuum and variability (Bregman 1990, 1994), on the properties of the gas in the inner regions of the AGN (Collin-Souffrin & Lasota 1988), on reverberation mapping of the emission line regions (Peterson 1993), and an overview of the AGN field (Blandford et al 1990).

The AGN that are the subject of this review are those in which the central optical, UV, and X-ray source and the broad emission line region (if present) are viewed directly. The word AGN is used here regardless of redshift and luminosity and therefore encompasses the words Seyfert 1 and QSO or Quasar, which are often used to designate low- and high-luminosity AGN separately. In radio-loud AGN seen at small angles to the axis of the jet, the highly nonthermal radiation produced in the jet is strongly amplified by relativistic beaming and dominates the observed continuum. In these sources, called blazars, variability is the most violent and affects the whole electromagnetic range from the radio to the gamma-ray band.

The fundamentally different character of the radiation emitted by radio-quiet AGN and by blazars dictates different observational goals and techniques. For radio-quiet AGN the focus is on (a) the emission mechanisms of the optical-UV-X-ray continuum and (b) the kinematics of the gas, with the ultimate aim of investigating the mass accretion and mass loss, and of deriving the mass of the central black hole if, indeed, it can be shown that the kinematics is dominated by virialized motions. For blazars, the goal is to understand the structure and physical state of the plasma in the jet, i.e. the geometry, acceleration, and radiation processes.

Accordingly, this review is organized into two main parts, radio-quiet AGN (Sections 2 - 5) and blazars (Sections 6 - 8), with general conclusions in Section 9.