Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A dedicated Chandra ACIS observation of the central compact object in
  the Cassiopeia A supernova remnant   We present results of a recent Chandra X-ray Observatory observation of the
central compact object (CCO) in the supernova remnant Cassiopeia A. This
observation was obtained in an instrumental configuration that combines a high
spatial resolution with a minimum spectral distortion, and it allowed us to
search for pulsations with periods longer than 0.68 s. We found no evidence of
extended emission associated with the CCO, nor statistically significant
pulsations (the 3-sigma upper limit on pulsed fraction is about 16%). The fits
of the CCO spectrum with the power-law model yield a large photon index,
Gamma\approx 5, and a hydrogen column density larger than that obtained from
the SNR spectra. The fits with the blackbody model are statistically
unacceptable. Better fits are provided by hydrogen or helium neutron star
atmosphere models, with the best-fit effective temperature kT_{eff}^\infty
\approx 0.2 keV, but they require a small star's radius, R = 4 - 5.5 km, and a
low mass, M < 0.8 M_sol. A neutron star cannot have so small radius and mass,
but the observed emission might emerge from an atmosphere of a strange quark
star. More likely, the CCO could be a neutron star with a nonuniform surface
temperature and a low surface magnetic field (the so-called anti-magnetar),
similar to three other CCOs for which upper limits on period derivative have
been established. The bolometric luminosity, L_{bol}^\infty \sim 6\times
10^{33} erg s^{-1}, estimated from the fits with the hydrogen atmosphere
models, is consistent with the standard neutron star cooling for the CCO age of
330 yr. The origin of the surface temperature nonuniformity remains to be
understood; it might be caused by anisotropic heat conduction in the neutron
star crust with very strong toroidal magnetic fields.

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The categories of this paper are: comp-gas
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: econ
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The categories of this paper are: eess
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The categories of this paper are: stat
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The categories of this paper are: cmp-lg
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The categories of this paper are: q-alg
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The categories of this paper are: ao-sci
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The categories of this paper are: physics
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The categories of this paper are: q-fin
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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: plasm-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: mtrl-th