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

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VARIABILITY OF ACTIVE GALACTIC NUCLEI

Marie-Helene Ulrich


European Southern Observatory, Karl-Schwarzschild Strasse 2, D-85748 Garching bei München, FRG

Laura Maraschi


Brera Astronomical Observatory, via Brera 28, 20121 Milan, Italy

C. Megan Urry

Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA


KEY WORDS: Seyfert galaxies, quasars, blazars, black holes, emission lines, accretion disks, jets


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

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