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Prior to about 1988, there were a large number of observations that suggested that the broad emission lines in Seyferts varied in response to continuum variations and did so on surprisingly short time scales. These early results led to the first highly successful reverberation campaign, carried out in 1988 - 89, combining UV spectra obtained with the International Ultraviolet Explorer (IUE) with ground-based optical observations from numerous observatories. The program ran for over 8 months and achieved time resolution of a few days in several continuum and emission-line time series (Clavel et al. 1991; Peterson et al. 1991; Dietrich et al. 1993; Maoz et al. 1993). A number of important results were produced by this project, including:

  1. From the shortest measured wavelength (1350 Å) to the longest (5100 Å), the continuum variations appear to be in phase, with any lags between bands amounting to no more than a couple of days.

  2. The highest ionization emission lines respond most rapidly to continuum variations (e.g., ~ 2 days for He II lambda1640 and ~ 10 days for Lyalpha and C IV lambda1549) and the lower ionization lines respond less rapidly (e.g., ~ 20 days for Hbeta and nearly 30 days for C III] lambda1909). The BLR thus shows radial ionization stratification.

Optical spectroscopic monitoring of NGC 5548 continued for a total of 13 years, and during the fifth year of the program (1993), concurrent high-time resolution (daily observations) were made for about 60 days with IUE and for 39 days with the Faint Object Spectrograph on Hubble Space Telescope (Korista et al. 1995). Over time, it became clear that the Hbeta emission-line lag is a dynamic quantity, it varies with time and is dependent on the current mean continuum luminosity (Peterson et al. 2002). In other words, there is much more nuclear gas on scales of thousands of gravitational radii than previously thought: at any given time, most of the emission in any particular line arises primarily in that gas for which the physical conditions optimally produce that particular emission line (cf. the "locally optimized cloud" model of Baldwin et al. 1995).

Peterson et al. (2004) recently completed a comprehensive reanalysis of 117 independent reverberation mapping data sets on 37 AGNs, measuring emission-line lags, line widths, and black hole masses for all but two of these sources. Calibration of the reverberation-based mass scale, as embodied in the scaling factor f in eq. (1), is set by assuming that AGNs follow the same relationship between black hole mass and the host-galaxy bulge velocity dispersion (the MBH - sigma* relationship) seen in quiescent galaxies (Onken et al. 2004). The range of measured masses runs from ~ 2 × 106 Modot for the narrow-line Seyfert 1 galaxy NGC 4051 to ~ 1.3 × 109 Modot for the quasar PG 1426+015. The statistical errors in the mass measurements (due to uncertainties in lag and line-width measurement) are typically only about 30%. However, the systematic errors, due to scatter in the MBH - sigma* relationship, amount to about a factor of three; this systematic uncertainty can decreased only by understanding the geometry and kinematics of the BLR. Figure 5 shows a current version of the mass-luminosity relationship for AGNs, based on these reverberation-based black hole masses.

Figure 5

Figure 5. Black hole mass vs. luminosity for 35 reverberation-mapped AGNs. The luminosity scale on the lower x-axis is loglambda Llambda in units of ergs s-1. The upper x-axis shows the bolometric luminosity assuming that Lbol propto 9 lambda Llambda. The diagonal lines show the Eddington limit LEdd, 0.1 LEdd, and 0.01LEdd. The open circles represent narrow-line Seyfert 1 galaxies. From Peterson et al. (2004).

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