Steps Toward Determination of the Size and the Structure of the Broad-Line Region in Active Galactic Nuclei: VI. Variability of NGC 3783 from Ground-Based Data
Stirpe G.M., et al., 1994, ApJ 425, 609

The Seyfert 1 galaxy NGC 3783 was intensely monitored in several bands between 1991 December and 1992 August. This paper presents the results from the ground-based observations in the optical and near-IR bands, which complement the data set formed by the International Ultraviolet Explorer (IUE) spectra, discussed elsewhere. Spectroscopic and photometric data from several observatories were combined in order to obtain well-sampled light curves of the continuum and of H{beta}. During the campaign the source underwent significant variability. The light curves of the optical continuum and of H{beta} display strong similarities to those obtained with the IUE. The near-IR flux did not vary significantly except for a slight increase at the end of the campaign.

The cross-correlation analysis shows that the variations of the optical continuum have a lag of 1 day or less with respect to those of the UV continuum, with an uncertainty of <=4 days. The integrated flux of H{beta} varies with a delay of about 8 days. These results confirm that (1) the continuum variations occur simultaneously or with a very small lag across the entire UV-optical range, as in the Seyfert galaxy NGC 5548; and (2) the emission lines of NGC 3783 respond to ionizing continuum variations with less delay than those of NGC 5548. As observed in NGC 5548, the lag of H{beta} with respect to the continuum is greater than those of the high-ionization lines.

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