Multifrequency monitoring of the Seyfert 1 galaxy NGC 4593 - II. A small, compact nucleus?

We discuss the results of a campaign to monitor spectral variations in thelow-luminosity Seyfert~1 galaxy NGC~4593, at X-rays, ultraviolet, optical andnear IR frequencies. The observations and data analysis have been described ina companion paper (Santos-Lle\'o et al. 1994; Paper~1). The active nucleus inthis galaxy is strongly and rapidly variable in all wavebands, implying thatthe continuum source is unusually compact. Its energy distribution from1.2~$\mu$m to 1200~\AA\/ obeys a power-law whose index is significantly steeperthan is usual in Seyfert's or QSO's; the ``big bump'' is either absent orshifted to wavelengths shorter than 1200~\AA\/. The variations of thesoft-X~ray {\em excess\/} do not correlate with those of the UV or hard X-raycontinuum. The far UV and optical fluxes are well correlated, while thecorrelation between the hard X-rays and 1447 \AA\ continuum is only marginallysignificant. Moreover, the optical flux cannot lag behind the UV by more than 6days. These results cannot be accommodated in the framework of the standardgeometrically thin accretion disk model. Rather, they suggest that the bulk ofthe UV and optical flux originates from thermal reprocessing of X-raysirradiating the disk. The soft X-ray excess is probably the only spectralcomponent which originates from viscous dissipation inside the disk and thenear infrared is probably emitted by hot dust heated by the UV radiation. Sucha model is consistent with NGC~4593 having a relatively small black-hole massof the order of $2\times10^{6}{\rm M_{\odot}}$ as inferred from the linevariability study. The high ionization/excitation emission lines are very broadand strongly variable and their variations correlate with those of thecontinuum. The low excitation lines are significantly narrower and remainconstant within the accuracy of our measurements. These results suggest astratified BLR, where the degree of ionization and the velocity dispersion ofthe gas increase toward small radii. The \lya\ line responds to the variationsof the continuum with a delay $\leq 4$ days. To a first order approximation,the BLR in NGC~4593 is well modelled with two different zones at distances of$\sim$~15 and 3 lt-ds from the ionizing source respectively.Comment: 36 pages, postscript, compressed. tables included; Figures available upon request. MNRAS in press