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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 $f_{obscured}thickapprox 0.42 (1+z)^{0.60}$.
Even in deep X-ray surveys, Compton-thick active galactic nuclei (CT AGNs, ${rm N_H} geqslant 1.5~times~10^{24}~{rm cm}^{-2}$) are difficult to be identified due to X-ray flux suppression and their complex spectral shape. However, the study of CT AGN
We present a detailed X-ray spectral analysis of 1152 AGNs selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs ($N_{rm H} > 10^{23} rm cm^{-2}$). By fitting spectra with physical models, 436 (38%) sources with $L_{rm
The high quality light curves of Kepler space telescope make it possible to analyze the optical variability of AGNs with an unprecedented time resolution. Studying the asymmetry in variations could give independent constraints on the physical models
We systematically analyze X-ray variability of active galactic nuclei (AGNs) in the 7~Ms textit{Chandra} Deep Field-South survey. On the longest timescale ($approx~17$ years), we find only weak (if any) dependence of X-ray variability amplitudes on e
We investigate the X-ray variability characteristics of hard X-ray selected AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data. The uncertainties involved in the individual dwell measurements of ASM are critically examined