X-ray spectral analyses of AGNs from the 7Ms Chandra Deep Field-South
  survey: the distribution, variability, and evolution of AGN's obscuration
  We present a detailed spectral analysis of the brightest Active Galactic
Nuclei (AGN) 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 K$\alpha$ 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 time
scales 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 re-selected subsamples which 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 that from C-COSMOS, we confirm the anti-correlation 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 $f_{obscured}\thickapprox 0.42\ (1+z)^{0.60}$.
