Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Chandra survey of the SMC "Bar": II. Optical counterparts of X-ray
  sources   We present the most likely optical counterparts of 113 X-ray sources detected
in our Chandra survey of the central region of the Small Magellanic Cloud (SMC)
based on the OGLE-II and MCPS catalogs. We estimate that the foreground
contamination and chance coincidence probability are minimal for the bright
optical counterparts (corresponding to OB type stars; 35 in total). We propose
here for the first time 13 High-Mass X-ray Binaries (HMXBs), of which 4 are
Be/X-ray binaries (Be-XRBs), and we confirm the previous classification of 18
Be-XRBs. We estimate that the new candidate Be-XRBs have an age of 15-85 Myr,
consistent with the age of Be stars. We also examine the "overabundance" of
Be-XRBs in the SMC fields covered by Chandra, in comparison with the Galaxy. In
luminosities down to about 10^{34} erg/s, we find that SMC Be-XRBs are 1.5
times more common when compared to the Milky Way even after taking into account
the difference in the formation rates of OB stars. This residual excess can be
attributed to the lower metallicity of the SMC. Finally, we find that the
mixing of Be-XRBs with other than their natal stellar population is not an
issue in our comparisons of Be-XRBs and stellar populations in the SMC.
Instead, we find indication for variation of the SMC XRB populations on
kiloparsec scales, related to local variations of the formation rate of OB
stars and slight variation of their age, which results in different relative
numbers of Be stars and therefore XRBs.

27
The categories of this paper are: comp-gas
33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
18
The categories of this paper are: econ
20
The categories of this paper are: acc-phys
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess