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 NH, observed fluxes, and absorption-corrected 2-10 keV luminosities LX for our sample of AGNs, but also identify significant spectral variabilities among them on timescales of years. We find that the NH 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 NH can be well explained by the continuum suppression with increasing NH. Accounting for the sample incompleteness and bias, we measure the intrinsic distribution of NH for the CDF-S AGN population and present reselected subsamples that are complete with respect to NH. The NH-complete subsamples enable us to decouple the dependences of NH on LX and on redshift. Combining our data with those from C-COSMOS, we confirm the anticorrelation between the average NH and LX of AGN, and find a significant increase of the AGN-obscured fraction with redshift at any luminosity. The obscured fraction can be described as fobscured ≈ 0.42 (1+z)0.60.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science