Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Most hard X-ray selected quasars in Chandra Deep Fields are obscured   Measuring the population of obscured quasars is one of the key issues to
understand the evolution of active galactic nuclei (AGNs). With a redshift
completeness of 99%, the X-ray sources detected in Chandra Deep Field South
(CDF-S) provide the best sample for this issue. In this letter we study the
population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected
sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV
band selected samples also filter out most of the X-ray faint sources with too
few counts, for which the measurements of N_H and L_X have very large
uncertainties. Simply adopting the best-fit L_2-10keV and N_H, we find 71% (20
out of 28) of the quasars (with intrinsic L_2-10keV > 10^44 erg/s) are obscured
with N_H > 10^22 cm^-2. Taking account of the uncertainties in the measurements
of both N_H and L_X, conservative lower and upper limits of the fraction are
54% (13 out 24) and 84% (31 out 37). In Chandra Deep Field North, the number is
29%, however, this is mainly due to the redshift incompleteness. We estimate a
fraction of ~ 50% - 63% after correcting the redshift incompleteness with a
straightforward approach. Our results robustly confirm the existence of a large
population of obscured quasars.

9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
30
The categories of this paper are: q-alg
18
The categories of this paper are: econ
3
The categories of this paper are: physics
35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex
14
The categories of this paper are: math-ph
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
36
The categories of this paper are: cmp-lg