Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
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Before reviewing the progress realized by the study of line variability, we briefly recall two critically important results obtained from spectroscopy alone. First, the high- and the low-ionization lines (HIL and LIL) are emitted by two different gas phases (Kwan & Krolik 1981, Collin-Souffrin et al 1986, Netzer 1987). The LIL come from a very dense medium with Ne geq 1011 cm-3, an ionization parameter much less than 0.1, and a column density exceeding 1024 cm-2. This medium must have a flat geometry, and there is the attractive possibility that the LIL are emitted by the accretion disk (Collin-Souffrin 1987). The HIL come from a more dilute medium, possibly a wind (van Groningen 1987, Collin-Souffrin & Lasota 1988) or an ensemble of clouds in a broad cone above and below the disk, with the gas density in the clouds not larger than a few 1010 cm-3 and an ionization parameter of the order of 0.3.

Second, the gas velocity and its degree of ionization are correlated. Among the HIL, the broadest FWHM and the most extensive wings are those of the most highly ionized species (NGC 5548: Krolik et al 1991; NGC 4151: Antonucci & Cohen 1983, Ulrich et al 1984a). Similarly, in a few well observed AGN the Hbeta; line has more extended wings than Halpha. (We recall the nomenclature in this context: the main LIL are the Balmer lines, optical and UV Fe II multiplets, Mg IIlambda2796,2803, He Ilambda5876. Among the HIL, the strongest are C IVlambda1548,1551, then C III]lambda1909, Si IVlambda1394,1403, He IIlambda4686, N Vlambda1239,1243, and Fe[X]lambda6375. The Lyalpha line is special and has an intensity about twice that of like C IV).

The crucial element contributed by variability studies is the linear scale of the LIL and HIL emission regions. For well-sampled high S/N data, the three-dimensional architecture and velocity field of the gas clouds can be reconstructed from the velocity-resolved transfer function. The results are at present dominated by the two bright, well-observed, strongly variable and intrinsically weak AGN, NGC 4151 and NGC 5548, and by 3C 273. Other AGN give consistent results. Most of the observational data on line variability come from the sources quoted for the continuum data in Section 2.

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