Steps Toward Determination of the Size and Structure of the Broad-Line   Region in Active Galactic Nuclei. XV. Long-Term Optical Monitoring of NGC   5548
  We present the results of three years of ground-based observations of the Seyfert 1 galaxy NGC 5548, which combined with previously reported data, yield optical continuum and broad-line H-beta light curves for a total of eight years. The light curves consist of over 800 points, with a typical spacing of a few days between observations. During this eight-year period, the nuclear continuum has varied by more than a factor of seven, and the H-beta emission line has varied by a factor of nearly six. The H-beta emission line responds to continuum variations with a time delay or lag of 10-20 days, the precise value varying somewhat from year to year. We find some indications that the lag varies with continuum flux in the sense that the lag is larger when the source is brighter. 
B.M. Peterson, A.J. Barth, P. Berlind, R. Bertram, K. Bischoff, N.G.   Bochkarev, A.N. Burenkov, F.-Z. Cheng, M. Dietrich, A...