Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Missing X-ray Background   The fraction of the hard X-ray background (XRB) resolved into individual
sources by the deep Chandra and XMM-Newton surveys strongly depends on the
adopted energy range and decreases with increasing energy. As a consequence,
the nature of the sources of the even harder (>10 keV) XRB remains
observationally poorly constrained. I will briefly discuss the need for X-ray
observations above 10 keV.

33
The categories of this paper are: plasm-ph
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
23
The categories of this paper are: solv-int
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
2
The categories of this paper are: math
32
The categories of this paper are: atom-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: adap-org
15
The categories of this paper are: nucl-ex
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The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
8
The categories of this paper are: hep-ex