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 ${N}_{{\rm{H}}}$, 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 ${N}_{{\rm{H}}}$ 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 ${N}_{{\rm{H}}}$ can be well explained by the continuum suppression with increasing ${N}_{{\rm{H}}}$. Accounting for the sample incompleteness and bias, we measure the intrinsic distribution of ${N}_{{\rm{H}}}$ for the CDF-S AGN population and present reselected subsamples that are complete with respect to ${N}_{{\rm{H}}}$. The ${N}_{{\rm{H}}}$-complete subsamples enable us to decouple the dependences of ${N}_{{\rm{H}}}$ on L X and on redshift. Combining our data with those from C-COSMOS, we confirm the anticorrelation between the average ${N}_{{\rm{H}}}$ 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 ${f}_{\mathrm{obscured}}\approx 0.42\ {(1+z)}^{0.60}$.