Steps toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. IV. Intensity Variations of the Optical Emission Lines of NGC 5548

We present measurements of optical emission-line flux variations based on spectra of the Seyfert galaxy NGC 5548 obtained between 1988 December and 1989 October during the course of a large-scale international monitoring effort. The data presented here supplement previously published measurements of the UV lines and continuum, optical continuum, and broad Hβ emission line. All of the measured optical emission lines, Hα, Hβ, Hγ, He I λ5876, and He II λ4686, show the same qualitative behavior as the UV and optical continua, but with short time delays, or lags, which are different for the various lines. We apply cross-correlation analysis to measure the lags between the various lines and the continuum. We find similar lags with respect to the UV continuum for Hα and Hβ, 17 and 19 days, respectively. The lag for Hγ is shorter (13 days), only somewhat larger than the lag measured for Lyα (about 10 days). The helium lines respond to continuum variations more rapidly than the hydrogen lines, with lags of about 7 days for He II λ4686 and 11 days for He I λ5876.