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 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 $L_{\rm X} > 10^{42}\ \rm erg\ s^{-1}$ are confirmed to be highly
obscured, including 102 Compton-thick (CT) candidates. We propose a new
hardness-ratio measure of the obscuration level which 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 logN-logS
relation favors cosmic X-ray background models that predict moderate (i.e.,
between optimistic and pessimistic) CT number counts. 19% (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 sightline may obscure the compact X-ray emitter. After correcting for
several observational biases, we obtain the intrinsic NH distribution and its
evolution. The CT-to-highly-obscured fraction is roughly 52% and is consistent
with no evident redshift evolution. We also perform long-term (~17 years 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 NH, 53% (9/17) are caused by the intrinsic
luminosity variability, 6% (1/17) are driven by both effects, and 2 are not
classified due to large spectral fitting errors.
