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
Dietrich M., et al., 1993, ApJ 408, 416

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 H{beta} emission line. All of the measured optical emission lines, H{alpha}, H{beta}, H{gamma}, He I {lambda}5876, and He II {lambda}4686, 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 H{alpha} and H{beta}, 17 and 19 days, respectively. The lag for H{gamma} is shorter (13 days), only somewhat larger than the lag measured for Ly{alpha} (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 {lambda}4686 and 11 days for He I {lambda}5876.

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