ON THE EMISSION-LINE RESPONSE TO CONTINUUM VARIATIONS IN THE SEYFERT-GALAXY NGC-5548

Recently, two intensive optical monitoring projects were carried out, with the aim of measuring the continuum and emission line variations in the Seyfert galaxy NGC 5548. The two groups, observing the galaxy during two different periods, reported apparently contradicting results for the delay of Hβ variations with respect to the continuum. Using the UV measurements of Clavel et al., obtained the same year as one of the above data sets, we show that the emission-line lag behind continuum variations depends on the continuum variability time scale in this object, in the sense that continuum variations with larger time scales give larger emission-line lags. Since the continuum variability time scales are different in the data sets obtained by the two groups, so are the derived lags. Such an effect can arise in some thick BLR geometries, especially if the line response to the ionizing flux changes is nonlinear. Using Monte Carlo simulations, we show that even without resorting to a nonlinear response, there is at least one possible model that could reproduce the two differing delays. In view of the effect, we note that previously reported differences in the lags of various emission lines are uncertain. In particular, we find that, using only the largest outburst in the simultaneous optical and UV light curves, Lyα, C IV λ1549, and Hβ all have similar lags behind the continuum.