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

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

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ABSTRACT. A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X ray source. The validity of this model for high luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The "accretion disk plus wind" model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined.

In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The general correlation found between variations in TeV gamma rays and in X rays for Mrk 421, and between variations in GeV gamma rays and in the IR-optical-UV bands for 3C 279, two prototype objects, supports models in which the same population of relativistic electrons radiates the low frequency continuum via synchrotron and the high frequency continuum via inverse Compton scattering of soft photons. Identifying the dominant source of soft photons, which is at present unclear, will strongly constrain the jet physics.

Table of Contents

INTRODUCTION

VARIABILITY OF THE CONTINUUM IN LOW REDSHIFT RADIO-QUIET AGN

Continuum Variability in the Optical, UV, EUV, and IR Ranges

Variability of the X-Ray Emission of Seyfert Galaxies

Simultaneity of the Flux Variations at Various Energies

VARIABILITY OF THE OPTICAL CONTINUUM IN HIGH REDSHIFT AGN

Observed Characteristics of the Optical Variability

Microlensing as a Possible Cause of Quasar Variability

EMISSION LINE VARIABILITY: RATIONALE AND METHODS

Rationale for Emission Line Variability Study

Inversion Methods, Cross-Correlations, and Modeling

EMISSION LINE VARIABILITY: RESULTS

Results of Variability: The Stratification of the Broad Line Region in Velocity and Degree of Ionization

Mapping the Velocity Field from the Emission Lines

Other Issues Concerning the BLR

Summary, Perspectives, and Emerging Fields in Emission Line Variability Studies

VARIABILITY OF BLAZARS

Overview and Relativistic Beaming

Spectral Shape and Variability of the Blazar Continuum

Far IR-Optical-UV: The Thin Synchrotron Emission

X Rays: The Crossing of Different Emission Components

High Energy Gamma Rays: Where the Action Is

Periodicity of OJ 287

Variability of Emission Lines

MULTIWAVELENGTH STUDIES OF BLAZARS

Broadband Continuum Snapshots

Multiwavelength Light Curves and Correlations

Intensive Multiwavelength Campaigns

INTERPRETATION OF BLAZAR VARIABILITY

Summary of Variability Results

The Relativistic Jet: Synchrotron Radiation from the Outer Regions

High-Frequency Synchrotron Emission: Energy Stratification?

Inverse Compton Models: The Gamma-Ray Jet

The Invisible Jet Core

CONCLUSIONS AND FUTURE INVESTIGATIONS

REFERENCES