Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. II. An intensive study of NGC 5548 at optical wavelengths

We report on a large, international program of ground-based optical spectroscopy and photometry of the variable Seyfert 1 galaxy NGC 5548 undertaken in support of an IUE monitoring campaign described by Clavel and coworkers. In this contribution, we present the data base and describe the methods used to correct for systematic differences between spectra from different sources. Optical continuum and Hβ emission-line light curves are derived from the spectra. The behavior of the optical continuum is qualitatively the same as the behavior of the ultraviolet continuum. Cross-correlation of the ultraviolet and optical continuum measurements does not reveal any significant lag between them. The Hβ emission-line variations show the same basic pattern as seen in the continuum and ultraviolet emission lines, with Hβ lagging behind the continuum by ∼20 days. This is significantly larger than the ∼10 day lag deduced for Lyα.