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Article Contents

ABSTRACT

1.INTRODUCTION

2.BACKGROUND AND BASIC PHENOMENOLOGY

2.1Basic Characteristics of Variability

2.2.Origin of the Variations

2.3.Blazar Variability

2.4.Emission-Line Variability

2.5.The First Monitoring Programs

2.6.Spectrophotometric Flux Calibration

2.7.Requirements for a Successful Monitoring Program

3.THEORY OF REVERBERATION MAPPING

3.1.Reverberation Mapping Assumptions

3.2.The Transfer Equation

3.3.Isodelay Surfaces

3.4.Transfer Functions for a Variety of Simple Models

3.5.Transfer Function Recovery

4.CROSS-CORRELATION ANALYSIS

4.1.Fundamentals

4.2.Relationship to the Transfer Function

4.3.Discrete Correlation Methods

4.4.Computational Issues

4.5.Uncertainties in Cross-Correlation Lags

5.OBSERVATIONAL RESULTS

5.1.Size of the Broad-Line Region

5.2.AGN Black-Hole Masses

5.3.Emission-Line Transfer Functions

5.4.Emission-Line Profile Variability

5.5.Multiwavelength Continuum Variability

5.6.Conclusion