Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: X-ray sources in Galactic old open star clusters   I review the current status of studies of the X-ray sources in Galactic old
open clusters. Cataclysmic variables (CVs), magnetically-active binaries (ABs),
and sub-subgiants (SSGs) dominate the X-ray emission of old open clusters.
Surprisingly, the number of ABs detected inside the half-mass radius with Lx >~
1e30 erg/s (0.3-7 keV) does not appear to scale with cluster mass. Comparison
of the numbers of CVs, ABs, and SSGs per unit mass in old open and globular
clusters shows that each of these classes is under-abundant in globulars. This
suggests that dense environments suppress the frequency of even some of the
hardest binaries.

20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
19
The categories of this paper are: eess
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
30
The categories of this paper are: q-alg
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
3
The categories of this paper are: physics
32
The categories of this paper are: atom-ph