From Fairall 9 to High-z Objects: Pattern of Change in Ionization Structure

A study of 79 IUE spectra of the Seyfert galaxy Fairall 9 shows a considerable change in the continuum shape which can be characterized by several sections. The flux at 3075 A changes in approximate proportion to the optical and X-ray flux, suggesting that they may be a part of the power law. The apparent fourfold change in the continuum slope between 1826 and 3075 A indicates a considerably different variations for the UV bump and the power-law continuum. The most variable part between 1338 and 1826 A keeps a constant slope. Significant differential variations are found in weaker emission lines, including N v λ1240, Si IV λ1400, He II λ1640, and C III] λ1909. Low-ionization lines (threshold energy between 1 and 3 Ry) appear to follow the UV bump, and high-ionization lines follow the power-law continuum. The results, in addition to the previous report on strong lines, suggest that the variation amplitude of lines is inversely correlated to the effective excitation level. Our multiline fit suggests that the UV bump has a shape more complicated than a simple black-body curve and its high-energy tail extends beyond 3 eV. An active galactic nucleus statistic finds that the degree of luminosity dependence of equivalent width is inversely correlated with individual emission line's excitation level. This offers a natural explanation as to why the C IV equivalent width shows the most significant luminosity dependence (the Baldwin effect). We suggest a more prominent UV bump in high states or in high-luminosity objects.