HIGH-ENERGY SPECTRA OF ACTIVE GALACTIC NUCLEI .2. ABSORPTION IN SEYFERT-GALAXIES

Absorption by cold material in a large sample of active galaxies has b een analyzed in order to study statistically the behavior of absorbed sources. The analysis indicates that on the basis of the column densit y alone, sources can be divided into low-absorption ([N-H/N-HGa1] less than or equal to 50) and high-absorption ([N-H/N-HGa1] greater than o r equal to 50) objects. While the second group consists mostly of narr ow emission line galaxies (Seyfert galaxies of type 1.9-2), the first group is less homogenous, being formed by a mixture of broad and narro w emission line objects (Seyfert 1-2 galaxies). A study of the distrib ution of the column density values by means of bootstrap analysis conf irms the reality of this effect. One group consisting of optically sel ected objects is well explained within the unified theory as nuclei ob scured by a molecular torus. The second group made up of X-ray-and IRA S-selected objects is more difficult to define: in these sources the a bsorption is underestimated owing to difficulties (1) in fitting compl ex absorption spectra or (2) in measuring N-H values in Compton-thick sources or the absorption has a different origin than in the torus. Po ssible correlations of absorption with X-ray luminosity, axial ratio, and Balmer decrement have also been investigated. Previous suggestions that lower luminosity AGNs tend to be more highly absorbed than those with higher luminosity are not confirmed by the present data; neither is any evidence for a correlation of N-H with axial ratio (b/a) found except for a preference of Seyfert 1-1.5 galaxies to be in face-on ga laxies. While some sources (Seyfert 1-1.5 galaxies and low-absorption objects) have X-ray absorption compatible with Balmer decrement, high- absorption objects have column densities much higher than predicted fr om optical observations. These results are in agreement with the unifi ed theory since the torus parameters are expected to be independent of luminosity, its orientation should be random with respect to the host galaxy, and its location should be in between the broad- and narrow-l ine regions. A study of the N-H variability indicates that in a large fraction (70%) of the sources for which the analysis could be done, N- H varies on timescales from months to years. In Seyfert 1-1.5 galaxies , the variability is associated with a region in or near the broad-lin e region and is explained in terms of partial covering and/or warm abs orption models. In Seyfert 2 galaxies, the only variability observed i s that associated with narrow emission line galaxies. The study of the column density distributions indicates that Seyfert 1-1.5 galaxies ar e characterized by N-H = 18(-7)(+9) x 10(21) atoms cm(-2). Seyfert 1.9 -2 galaxies have instead N-H = 96(-35)(+54) x 10(21) atoms cm(-2) and a larger dispersion; if this group is divided into low- and high-absor ption objects, N-H = 14.5(-5.3)(+7.2) x 10(21) atoms cm(-2) and N-H = 132.8(-52.6)(+80.1) x 10(21) atoms cm(-2), respectively, are obtained. The observed dispersion in each group is consistent with being entire ly due to column density variability.