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-ray, ultraviolet, optical and near-IR frequencies. The observations and data analysis have been described in a companion paper. 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 μm to 1200A obeys a power law which is significantly steeper than is usual in Seyferts or QSOs; the 'big bump' is either absent or shifted to wavelengths shorter than 1200A. The variations of the soft X-ray 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-A continuum is only marginally significant. Moreover, the optical flux cannot lag behind the UV by more than 6d. These results cannot be accommodated in the framework of the standard geometrically thin accretion disc model. Rather, they suggest that the bulk of the UV and optical flux originates from thermal reprocessing of X-rays irradiating the disc. The soft X-ray excess is probably the only spectral component that originates from viscous dissipation inside the disc, 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 2x10^6 M_solar 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 broad-line region, where the degree of ionization and the velocity dispersion of the gas increase toward small radii. The Lyalpha lambda1216 line responds to the variations of the continuum with a delay of <=4d. To a first-order approximation, the broad-line region in NGC 4593 is well modelled by two different zones at distances of ~15 and 3 light-day from the ionizing source respectively.