STEPS TOWARD DETERMINATION OF THE SIZE AND STRUCTURE OF THE BROAD-LINE REGION IN ACTIVE GALACTIC NUCLEI .5. VARIABILITY OF THE ULTRAVIOLET CONTINUUM AND EMISSION-LINES OF NGC-3783

We report on the result of intensive ultraviolet spectral monitoring o f the Seyfert 1 galaxy NGC 3783. The nucleus of NGC 3783 was observed with the International Ultraviolet Explorer satellite on a regular bas is for a total of 7 months, once every 4 days for the first 172 days a nd once every other day for the final 50 days. Significant variability was observed in both continuum and emission-line fluxes. The light cu rves for the continuum fluxes exhibited two well-defined local minima or ''dips,'' the first lasting less than or similar to 20 days and the second less than or similar to 4 days, with additional episodes of re latively rapid flickering of approximately the same amplitude. As in t he case of NGC 5548 (the only other Seyfert galaxy that has been the s ubject of such an intensive, sustained monitoring effort), the largest continuum variations were seen at the shortest wavelengths, so that t he continuum became ''harder'' when brighter. The variations in the co ntinuum occurred simultaneously at all wavelengths (Delta t <2 days). Generally, the amplitude of variability of the emission lines was lowe r than (or comparable to) that of the continuum. Apart from Mg II (whi ch varied little) and N V (which is relatively weak and badly blended with Ly alpha), the light curves of the emission lines are very simila r to the continuum light curves, in each case with a small systematic delay or ''lag'' As for NGC 5548, the highest ionization lines seem to respond with shorter lags than the lower ionization lines. The lags f ound for NGC 3783 are considerably shorter than those obtained for NGC 5548, with values of(formally) similar to 0 days for He II + O III], and similar to 4 days for Ly alpha and C IV. The data further suggest lags of similar to 4 days for Si IV + O IV] and 8-30 days for Si III] + C III]. Mg II lagged the 1460 Angstrom continuum by similar to 9 day s, although this result depends on the method of measuring the line fl ux and may in fact be due to variability of the underlying Fe II lines . Correlation analysis further shows that the power density spectrum c ontains substantial unresolved power over timescales of less than or s imilar to 2 days, and that the character of the continuum variability may change with time.