Modeling Variable Emission Lines in Active Galactic Nuclei: Method and Application to NGC 5548

We present a new scheme for modeling the broad line region in active galacticnuclei (AGNs). It involves photoionization calculations of a large number ofclouds, in several pre-determined geometries, and a comparison of thecalculated line intensities with observed emission line light curves. Fittingseveral observed light curves simultaneously provides strong constraints onmodel parameters such as the run of density and column density across thenucleus, the shape of the ionizing continuum, and the radial distribution ofthe emission line clouds. When applying the model to the Seyfert 1 galaxy NGC5548, we were able to reconstruct the light curves of four ultravioletemission-lines, in time and in absolute flux. This has not been achieved by anyprevious work. We argue that the Balmer lines light curves, and possibly alsothe MgII2798 light curve, cannot be tested in this scheme because of thelimitations of present-day photoionization codes. Our fit procedure can be usedto rule out models where the particle density scales as r^{-2}, where r is thedistance from the central source. The best models are those where the densityscales as r^{-1} or r^{-1.5}. We can place a lower limit on the column densityat a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit theparticle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. Wehave also tested the idea that the spectral energy distribution (SED) of theionizing continuum is changing with continuum luminosity. None of thevariable-shape SED tried resulted in real improvement over a constant SED casealthough models with harder continuum during phases of higher luminosity seemto fit better the observed spectrum. Reddening and/or different compositionseem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication in Ap