Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: X-rays from the Dawn of the Modern Universe. Chandra and XMM-Newton
  Observations of z>4 Quasars   Quasars at z>4 provide direct information on the first massive structures to
form in the Universe. Recent ground-based optical surveys (e.g., the Sloan
Digital Sky Survey) have discovered large numbers of high-redshift quasars,
increasing the number of known quasars at z>4 to ~500. Most of these quasars
are suitable for follow-up X-ray studies. Here we review X-ray studies of the
highest redshift quasars, focusing on recent advances enabled largely by the
capabilities of Chandra and XMM-Newton. Overall, analyses indicate that the
X-ray emission and broad-band properties of high-redshift and local quasars are
reasonably similar, once luminosity effects are taken into account. Thus,
despite the strong changes in large-scale environment and quasar number density
that have occurred from z~0-6, individual quasar X-ray emission regions appear
to evolve relatively little.

15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
37
The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
11
The categories of this paper are: hep-lat
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
32
The categories of this paper are: atom-ph
12
The categories of this paper are: nucl-th