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