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With the background provided in the previous sections, we can discuss some of the more important observational results. We will begin with emission-line variability, since the results obtained thus far have been relatively less ambiguous than the results on interband continuum variations.

5.1. Size of the Broad-Line Region

By the late 1980s, the potential power of reverberation mapping was widely understood, but the high-intensity monitoring programs required to extract this information had not been carried out, mainly on account of sociological barriers: the sampling requirements, in terms of time criticality and number of observations, were far in excess of anything that had been done in the past on oversubscribed shared facilities. In late 1987, a large informal consortium known as the International AGN Watch (a) was formed in an attempt to obtain sufficient observing time with IUE and various ground-based observatories to measure emission-line lags for the Seyfert 1 galaxy NGC 5548, even at that time one of the best-studied variable AGNs. This turned out to be an enormously successful project 14, 66, 67 that provided impetus for additional similar projects by the AGN Watch and other groups (see Peterson 62 for a review of the field in the wake of the first monitoring project on NGC 5548). Some of the light curves and CCFs from the original NGC 5548 project are shown in Fig. 31.

Figure 31

Figure 31. Light curves (left-hand column) and CCFs (right-hand column) for NGC 5548, from the first International AGN Watch monitoring project in 1988-89. From the top, the light curves are the UV continuum at 1350 Å, the optical continuum at 5100 Å, He II lambda1640, Lyalpha lambda1215, C IV lambda1549, C III] lambda1909, and Hbeta lambda4861. The continuum fluxes are in units of 10-15 ergs s-1 cm-2 Å-1, and the line fluxes are in units of 10-13 ergs s-1 cm-2. The CCFs are computed relative to the UV continuum at the top; the top panel is the UV continuum ACF. Data from Clavel et al. 14 and Peterson et al. 67 .

Since this initial successful campaign, a handful of AGNs have been well-monitored in the UV and optical (NGC 3783, NGC 4151, NGC 7469, 3C 390.3, Fairall 9, in addition to NGC 5548) and nearly three dozen Seyfert galaxies and low-luminosity quasars have been closely monitored in the optical 88, 36 . The most important general conclusions reached from these monitoring programs are:

  1. The UV/optical continua vary together, to within a couple of days.

  2. The highest ionization emission lines respond most rapidly to continuum changes, implying that there is ionization stratification of the BLR.

  3. The BLR gas is not in pure radial motion, since no unambiguous differences in the time scale of the response of the blue and red wings of the emission lines have been detected.

Cross-correlation results show that the size of the Hbeta-emitting region scales with average continuum luminosity as 36

Equation 38     (38)

and that, in general, sizes derived from other lines usually scale as r(He II, N V) approx 0.2 r(Hbeta) and r(Lyalpha, C IV}) approx 0.5 r(Hbeta). A slightly different version of the BLR radius-luminosity relationship is shown in Fig. 32, in which r propto L0.6 ± 0.1. This is close to the slope of the relationship predicted by the most naive sort of scaling: since to lowest order AGN spectra all look alike, the ionization parameters (Eq. (13)) and particle densities must be the same in all of them. Thus, inverting Eq. (13), we predict that

Equation 39     (39)

where in the last step we have also assumed that the shape of the ionizing continuum is not a function of luminosity. Despite these gross assumptions, the agreement with the data is not bad, although the large scatter alone tells us that this is not the whole story.

Figure 32

Figure 32. The BLR radius, as measured by the Hbeta lag, as a function of average optical continuum luminosity. The open symbols are Palomar-Green quasars from Kaspi et al. 36, the filled circles are Seyfert 1 galaxies from Wandel et al. 88. The large symbols are narrow-line (VFWHM < 2000 km s-1) AGNs, and the filled diamond is NGC 4051 75. The dotted line is the best fit to all the data except NGC 4051.

a More information about the International AGN Watch and all papers and data obtained are available on the AGN Watch website at Back.

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