THE SEYFERT-1 GALAXY NGC-4593 .2. THE PATTERN OF VARIABILITY OF THE UV SPECTRUM

IUE observations of NGC 4593 reveal ∼30 per cent variations in the intensities of the emission lines and UV continuum in 9 days. This sets upper limits to the dimension of the UV continuum and broad line emitting regions comparable to that found for NGC 4151 and 3783.UV observations at six different epochs reveal the existence of a strong correlation between (i) the UV continuum intensity and the power-law spectral index and (ii) the UV continuum intensity and the visual magnitude. There is also a general trend for the emission line intensities and widths to vary in the same sense as the continuum. However, with the exception of the H Lyα l216 line intensity and MG II 2800 linewidth, no significant correlations with the continuum flux emerge, probably owing to the finite dimension of the broad line region (BLR) and effects of time delays in its response to changes in the continuum.It is also found that lines from highly ionized species (e.g. C3+) have larger fractional range of variation than lines from less excited ions (e.g. Mg+). This, together with the finding that lines from different ions have different widths clearly proves that the BLR shows stratification effect. All data are consistent with the assumption that the degree of ionization and the velocity dispersion are decreasing functions of the distance to the central continuum source. © 1983 Royal Astronomical Society. Provided by the NASA Astrophysics Data System.