Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses

We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs ({N}_{{H}} > {10}²³ {cm}}^-2). By fitting spectra with physical models, 436 (38%) sources with {L}_{{X}}> {10}⁴² {erg} {{{s}}}^-1 are confirmed to be highly obscured, including 102 Compton-thick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N-log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic {N}_{{H}} distribution and its evolution. The CT/highly obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (≈17 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of {N}_{{H}}, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.