MULTIFREQUENCY MONITORING OF THE SEYFERT-1 GALAXY NGC-4593 .2. A SMALL, COMPACT NUCLEUS

We discuss the results of a campaign to monitor spectral variations in the low-luminosity Seyfert 1 galaxy NGC4593, at X-ray, ultraviolet, o ptical and near-IR frequencies. The observations and data analysis hav e been described in a companion paper. The active nucleus in this gala xy is strongly and rapidly variable in all wavebands, implying that th e continuum source is unusually compact. Its energy distribution from 1.2 mu m to 1200 Angstrom obeys a power law which is significantly ste eper than is usual in Seyferts or QSOs; the 'big bump' is either absen t or shifted to wavelengths shorter than 1200 Angstrom. The variations of the soft X-ray excess do not correlate with those of the UV or har d X-ray continuum. The far UV and optical fluxes are well correlated, while the correlation between the hard X-rays and 1447-Angstrom contin uum is only marginally significant. Moreover, the optical flux cannot lag behind the UV by more than 6 d. These results cannot be accommodat ed in the framework of the standard geometrically thin accretion disc model. Rather, they suggest that the bulk of the UV and optical flux o riginates from thermal reprocessing of X-rays irradiating the disc. Th e soft X-ray excess is probably the only spectral component that origi nates from viscous dissipation inside the disc, and the near-infrared is probably emitted by hot dust heated by the UV radiation. Such a mod el is consistent with NGC 4593 having a relatively small black hole ma ss of the order of 2x10(6) M. as inferred from the line variability st udy. The high-ionization/excitation emission lines are very broad and strongly variable, and their variations correlate with those of the co ntinuum. The low-excitation lines are significantly narrower and remai n constant within the accuracy of our measurements. These results sugg est a stratified broad-line region, where the degree of ionization and the velocity dispersion of the gas increase toward small radii. The L y alpha lambda 1216 line responds to the variations of the continuum w ith a delay of less than or equal to 4 d. To a first-order approximati on, the broad-line region in NGC 4593 is well modelled by two differen t zones at distances of similar to 15 and 3 light-day from the ionizin g source respectively.