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Paper: Accretion Disks and the Broad-Line Regions of Active Galactic Nuclei
Volume: 360, AGN Variability from X-Rays to Radio Waves
Page: 217
Authors: Eracleous, M.
Abstract: I review the evolution of ideas and models for the origin of the broad-emission lines of active galactic nuclei in gaseous structures associated with the accretion disk. I begin with early suggestions that gas launched from the disk is the source of the broad emission lines and I then move to more recent ideas attributing the line emission to the dense gas of the disk itself and to the base of the wind launched from it. I discuss the observational evidence supporting such models, including a comparison of observed emission line profiles from quasars with model predictions. I then move to a discussion of double-peaked emission lines, which, I argue, afford us a direct view of the disk and associated wind and serve as tools for studying their structure. I describe observations of the UV spectra of double-peaked emitters and interpret them in the context of the above models. Finally I address the issue of variability of double-peaked profiles by describing observational campaigns to characterize the variability and attempts to explain the results with the help of models of unsteady disks.
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