STEPS TOWARD DETERMINATION OF THE SIZE AND STRUCTURE OF THE BROAD-LINE REGION IN ACTIVE GALACTIC NUCLEI .7. VARIABILITY OF THE OPTICAL-SPECTRUM OF NGC-5548 OVER 4 YEARS

We report on the results of a continuation of a large monitoring progr am of optical spectroscopy of the Seyfert 1 galaxy NGC 5548. The new o bservations presented here were obtained between 1990 December and 199 2 October, and extend the existing database to nearly 1400 days, datin g back to 1988 December. The continuum variations are generally smooth and well-resolved, except during the third year of this 4 year projec t, when the variations were apparently more rapid and of lower amplitu de than observed at other times. The broad H beta emission line is fou nd to vary in response to the continuum variations with a lag of about Is days, but with some changes from year to year. The H beta transfer functions for each of the 4 yr and for the entire 4 yr database are d erived by using a maximum entropy method.