STEPS TOWARD DETERMINATION OF THE SIZE AND STRUCTURE OF THE BROAD-LINE REGION IN ACTIVE GALACTIC NUCLEI .8. AN INTENSIVE HST, IUE, AND GROUND-BASED STUDY OF NGC-5548

We present the data and initial results from a combined HST/IUE/ground -based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5 548 that was undertaken in order to address questions that require bot h higher temporal resolution and higher signal-to-noise ratios than we re obtained in our previous multiwavelength monitoring of this galaxy in 1988-1989. IUE spectra were obtained once every 2 days for a period of 74 days beginning on 1993 March 14. During the last 39 days of thi s campaign, spectroscopic observations were also made with the HST Fai nt Object Spectrograph (FOS) on a daily basis. Ground-based observatio ns, consisting of 165 optical spectra and 77 photometric observations( both CCD imaging and aperture photometry), are reported for the period 1992 October-1993 September, although many of the data are concentrat ed around the time of the satellite-based program. These data constitu te a fifth year of intensive optical monitoring of this galaxy. In thi s contribution we describe the acquisition and reduction of all of the satellite and ground-based data obtained in this program. We describe in detail various photometric problems with the FOS and explain how w e identified and corrected for various anomalies. During the HST porti on of the monitoring campaign, the 1350 Angstrom continuum flux is fou nd to have varied by nearly a factor of 2. In other wave bands, the co ntinuum shows nearly identical behavior, except that the amplitude of variability is larger at shorter wavelengths, and the continuum light curves appear to show more short-timescale variability at shorter wave lengths. The broad emission lines also vary in flux, with amplitudes t hat are slightly smaller than the UV continuum variations and with a s mall time delay relative to the UV continuum. On the basis of simple t ime-series analysis of the UV and optical continuum and emission-line light curves, we find (1)that the ultraviolet and optical continuum va riations are virtually simultaneous, with any lag between the 1350 Ang strom continuum and the 5100 Angstrom continuum amounting to less than about 1 day; (2)that the variations in the highest ionization lines o bserved, He II lambda 1640 and N V lambda 1240, lag behind the continu um variations by somewhat less than 2 days; and (3) that the velocity held of the C IV-emitting region is not dominated by radial motion. Th e results on the C Iv velocity held are preliminary and quite uncertai n, but there are some weak indications that the emission-line wings(\D elta upsilon\ greater than or equal to 3000 km s(-1)) respond to conti nuum variations slightly more rapidly than does the core. The optical observations show that the variations in the broad H beta line flux fo llow the continuum variations with a time lag of around 2 weeks, about twice the lag for Ly alpha and C Iv, as in our previous monitoring ca mpaign on this same galaxy. However, the lags measured for Ly alpha, C Iv, and H beta are each slightly smaller than previous determinations . We confirm two trends reported earlier, namely, (1) that the UV/opti cal continuum becomes ''harder'' as it gets brighter and (2) that the highest ionization emission lines have the shortest lags, thus indicat ing radial ionization stratification of a broad-line region that spans over an order of magnitude range in radius.