Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Can Chandra resolve the remaining cosmic X-ray background?   The deepest extragalactic X-ray observation, the 2 Ms Chandra Deep Field
North (CDF-N), resolves ~80% of the total extragalactic cosmic X-ray background
(CXB) in the 1-2 keV band. Recent work has shown that 70% of the remaining CXB
flux is associated with sources detected by the Hubble Space Telescope (HST).
This paper uses the existing CDF-N data to constrain the X-ray flux
distribution of these X-ray undetected HST sources, by comparing the number of
0.5-2 keV X-ray counts at the HST positions to those expected for model flux
distributions. In the simple case where all the undetected HST X-ray sources
have the same 0.5-2 keV flux, the data are best fit by 1.5-3 counts per source
in 2 Ms, compared to a detection limit (at 10% completeness) of 9 counts.
Assuming a more realistic power-law logN-logS distribution [N(>S) S^-alpha],
the data favor a relatively steep flux distribution, with alpha=1.1^+0.5_-0.3
(limits are 99% confidence). This slope is very similar to that previously
found for faint normal and starburst galaxies in the CDF-N. These results
suggest deeper Chandra observations will detect a new population of faint X-ray
sources, but extremely deep exposures are needed to resolve the remainder of
the soft CXB. In the most optimistic scenario, when the HST sources have the
flattest allowed flux distribution and all the sources without HST counterparts
are detected, observations 5 times more sensitive than the existing ones would
resolve at most ~60% of the remaining soft CXB.

19
The categories of this paper are: eess
8
The categories of this paper are: hep-ex
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
25
The categories of this paper are: patt-sol
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The categories of this paper are: q-fin
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The categories of this paper are: econ
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
30
The categories of this paper are: q-alg
26
The categories of this paper are: bayes-an
35
The categories of this paper are: mtrl-th