X-Ray Spectral Analyses of AGNs from the 7Ms Chandra Deep Field-South Survey: The Distribution, Variability, and Evolutions of AGN Obscuration

We present a detailed spectral analysis of the brightest active galactic nuclei (AGNs) identified in the 7Ms Chandra Deep Field-South (CDF-S) survey over a time span of 16 years. Using a model of an intrinsically absorbed power-law plus reflection, with possible soft excess and narrow Fe Kα line, we perform a systematic X-ray spectral analysis, both on the total 7Ms exposure and in four different periods with lengths of 2-21 months. With this approach, we not only present the power-law slopes, column densities N_H, observed fluxes, and absorption-corrected 2-10 keV luminosities L_X for our sample of AGNs, but also identify significant spectral variabilities among them on timescales of years. We find that the N_H variabilities can be ascribed to two different types of mechanisms, either flux-driven or flux-independent. We also find that the correlation between the narrow Fe line EW and N_H can be well explained by the continuum suppression with increasing N_H. Accounting for the sample incompleteness and bias, we measure the intrinsic distribution of N_H for the CDF-S AGN population and present reselected subsamples that are complete with respect to N_H. The N_H-complete subsamples enable us to decouple the dependences of N_H on L_X and on redshift. Combining our data with those from C-COSMOS, we confirm the anticorrelation between the average N_H and L_X of AGN, and find a significant increase of the AGN-obscured fraction with redshift at any luminosity. The obscured fraction can be described as f_obscured ≈ 0.42 (1+z)^0.60.