Invited Review. Published in The Publications of the Astronomical Society of the Pacific, Volume 111, pp. 1-30, 1999.

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THE ULTRAVIOLET AND OPTICAL CONTINUUM EMISSION IN ACTIVE GALACTIC NUCLEI: THE STATUS OF ACCRETION DISKS

ANURADHA KORATKAR


Space Telescope Science Institute, Baltimore, MD 21218; koratkar@stsci.edu

OMER BLAES


Physics Department, University of California, Santa Barbara, CA 93106; blaes@vela.physics.ucsb.edu


Abstract. A fundamental component of models of active galactic nuclei (AGNs) is an accretion disk around a supermassive black hole. However, the nature of this accretion disk is not well understood, and current models do not provide a satisfactory explanation of the optical/UV continuum observed in AGNs. In this paper we review the substantial theoretical and observational progress made in the field. We also try to point out future research directions that would be fruitful in trying to obtain a complete, self-consistent model of the continuum-emitting regions.


Table of Contents

INTRODUCTION

EARLY ACCRETION DISK MODELS

OBSERVATIONS
The Big Blue Bump
Lyman Edges
Polarization
Fe K alpha Emission Line and the Lyman Edge
Variability
Microlensing
Double-peaked Line Profiles

MORE RECENT ACCRETION DISK MODELS
The Big Blue Bump
The Lyman Edge
Polarization

ALTERNATIVES TO STANDARD ACCRETION DISKS
Thick Disks
Slim Disks
Advection-dominated Accretion Flows
Warped Disks
Optically Thin Emission and Multiphase Flows

SUMMARY AND DIRECTIONS FOR THE FUTURE

REFERENCES

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