2000ApJ...533..631K

Correlated variations in the line and continuum emission from active galactic nuclei (AGNs) can be used to determine the size and geometry of the broad emission-line regions (BLRs). We have spectrophotometrically monitored a well-defined sample of 28 Palomar-Green quasars in order to obtain measurements of their BLRs and to investigate the relationships between quasar luminosity, central black hole mass, and BLR size in AGNs. Spectrophotometry was obtained every 1-4 months for 7.5 yr, yielding 20-70 observing epochs per object. Both the continuum and emission-line fluxes of all of the quasars were observed to change during the duration of the observing program. Seventeen of the 28 objects were observed with adequate sampling (≳20 independent observing epochs) to search for correlated variations between the Balmer emission lines and the continuum flux. For each of these 17 objects, a significant correlation was observed, with the Balmer-line variations lagging those of the continuum by ∼100 days (rest frame). Our work increases the available luminosity range for studying the size-mass-luminosity relations in AGNs by 2 orders of magnitude and doubles the number of objects suitable for such studies. Combining our results with comparable published data available for Seyfert 1 galaxies, we find the BLR size scales with the rest-frame 5100 Å luminosity as L^0.70±0.03. This determination of the scaling of the size of the BLR as a function of luminosity is significantly different from those previously published and suggests that the effective ionization parameter in AGNs may be a decreasing function of luminosity. We are also able to constrain, subject to our assumption that gravity dominates the motions of the BLR gas, the scaling relationship between the mass of the central black holes and the luminosity in AGNs. We find that the central mass scales with 5100 Å luminosity as M∝L^0.5±0.1. This is inconsistent with all AGNs having optical luminosity that is a constant fraction of the Eddington luminosity.