The Seyfert 1 galaxy NGC 4593- II. The pattern of variability of theUV spectrum.

Abstract

IUE observations of NGC 4593 are found to show approximately 30 percent variations in the intensities of the emission lines and UV continuum in nine days, which set 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 show the existence of a strong correlation between the UV continuum intensity and the power-law spectral index as well as between the UV continuum intensity and the visual magnitude. The emission line intensities and widths also tend to vary in the same sense as the continuum. No significant correlations with the continuum flux are found, with the exception of the H Ly-alpha 1216 line intensity and MG II 2800 linewidth. In addition, it is determined that lines from highly ionized species such as C(3+) have larger fractional range of variation than lines from less excited ions such as Mg(+). This result together with the finding that lines from different ions have different widths proves that the broad line region shows stratification effect. These data indicate that the degree of ionization and the velocity dispersion are decreasing functions of the distance to the central continuum source.