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

Abstract

We discuss the results of a campaign to monitor spectral variations in the low-luminosity Seyfert~1 galaxy NGC~4593, at X-rays, ultraviolet, optical and near IR frequencies. The observations and data analysis have been described in a companion paper (Santos-Lle\'o et al. 1994; Paper~1). The active nucleus in this galaxy is strongly and rapidly variable in all wavebands, implying that the continuum source is unusually compact. Its energy distribution from 1.2~$\mu$m to 1200~\AA\/ obeys a power-law whose index is significantly steeper than is usual in Seyfert's or QSO's; the ``big bump'' is either absent or shifted to wavelengths shorter than 1200~\AA\/. The variations of the soft-X~ray {\em excess\/} do not correlate with those of the UV or hard X-ray continuum. The far UV and optical fluxes are well correlated, while the correlation between the hard X-rays and 1447 \AA\ continuum is only marginally significant. Moreover, the optical flux cannot lag behind the UV by more than 6 days. These results cannot be accommodated in the framework of the standard geometrically thin accretion disk model. Rather, they suggest that the bulk of the UV and optical flux originates from thermal reprocessing of X-rays irradiating the disk. The soft X-ray excess is probably the only spectral component which originates from viscous dissipation inside the disk and the near infrared is probably emitted by hot dust heated by the UV radiation. Such a model is consistent with NGC~4593 having a relatively small black-hole mass of the order of $2\times10^{6}{\rm M_{\odot}}$ as inferred from the line variability study. The high ionization/excitation emission lines are very broad and strongly variable and their variations correlate with those of the continuum. The low excitation lines are significantly narrower and remain constant within the accuracy of our measurements. These results suggest a stratified BLR, where the degree of ionization and the velocity dispersion of the gas increase toward small radii. The \lya\ line responds to the variations of 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.