Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hard X-rays from Ultra-Compact HII Regions in W49A   We report the Chandra detection of hard X-ray emission from the Welch ring in
W49A, an organized structure of ultra-compact (UC) HII regions containing a
dozen nascent early-type stars. Two UC HII regions are associated with hard
X-ray emission in a deep Advanced CCD Imaging Spectrometer image exposed for
96.7 ks. One of the two X-ray sources has no near-infrared counterpart and is
extended by ~5 arcsec, or ~0.3 pc, at a distance of ~11.4 kpc, which is
spatially aligned with the cometary radio continuum emission associated with
the UC HII region. The X-ray spectrum of the emission, when fit with a thermal
model, indicates a heavily absorbed plasma with extinction of
\~5x10^{23}/cm^{2}, temperature of ~7 keV, and X-ray luminosity in the 3.0-8.0
keV band of ~3x10^{33} ergs/s. Both the luminosity and the size of the emission
resemble the extended hard emission found in UC HII regions in Sagittarius B2,
yet they are smaller by an order of magnitude than the emission found in
massive star clusters such as NGC 3603. Three possibilities are discussed for
the cause of the hard extended emission in the Welch ring: an ensemble of
unresolved point sources, shocked interacting winds of the young O stars, and a
wind-blown bubble interacting with ambient cold matter.

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The categories of this paper are: econ
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The categories of this paper are: funct-an
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The categories of this paper are: chem-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: alg-geom
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The categories of this paper are: comp-gas
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The categories of this paper are: atom-ph
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The categories of this paper are: adap-org
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: cs
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The categories of this paper are: bayes-an
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The categories of this paper are: cmp-lg
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The categories of this paper are: plasm-ph
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The categories of this paper are: nucl-th