Broad-Line Variability in NGC 5548

We discuss the possible explanations for the variability behavior of the He II λ4686 and Hβ lines in the Seyfert galaxy NGC 5548 as observed by Peterson and Ferland. We explore the range of variability expected from these lines according to the standard models for broad-line clouds, assuming changes in the values of the clouds' defining parameters. When compared with model predictions, the observed line variability suggests the existence of a strong, variable EUV component of the ionizing spectrum and of two populations of clouds which differ in their velocity distributions and in their response to continuum variations.