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

Liu, Teng; Tozzi, Paolo; Wang, Jun-Xian; Brandt, William N.; Vignali, Cristian; Xue, Yongquan; Schneider, Donald P.; Comastri, Andrea; Yang, Guang; Bauer, Franz E.; Paolillo, Maurizio; Luo, Bin; Gilli, Roberto; Wang, Q. Daniel; Giavalisco, Mauro; Ji, Zhiyuan; Alexander, David M; Mainieri, Vincenzo; Shemmer, Ohad; Koekemoer, Anton; Risaliti, Guido

Abstract

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 ${f}_{\mathrm{obscured}}\approx 0.42\ {(1+z)}^{0.60}$.

Citation

Liu, T., Tozzi, P., Wang, J., Brandt, W. N., Vignali, C., Xue, Y., …Risaliti, G. (2017). X-Ray Spectral Analyses of AGNs from the 7Ms Chandra Deep Field-South Survey: The Distribution, Variability, and Evolutions of AGN Obscuration. Astrophysical Journal Supplement, 232(1), Article 8. https://doi.org/10.3847/1538-4365/aa7847

• Journal Article Type: Article
• Acceptance Date: Jun 6, 2017
• Online Publication Date: Aug 31, 2017
• Publication Date: Aug 31, 2017
• Deposit Date: Oct 12, 2017
• Publicly Available Date: Oct 12, 2017
• Journal: Astrophysical Journal Supplement
• Print ISSN: 0067-0049
• Electronic ISSN: 1538-4365
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 232
• Issue: 1
• Article Number: 8
• DOI: https://doi.org/10.3847/1538-4365/aa7847
• Public URL: https://durham-repository.worktribe.com/output/1346756