Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Supersoft X-ray Sources in M31: I. A Chandra Survey and an Extension to
  Quasisoft Sources   We report on very soft X-ray sources (VSSs) in M31. In a survey which was
most sensitive to soft sources in four 8'x8' regions covered by Chandra's
ACIS-S S3 CCD, we find 33 VSSs that appear to belong to M31. Fifteen VSSs have
spectral characteristics mirroring the supersoft X-ray sources studied in the
Magellanic Cloud and Milky Way ($k T_{eff} \leq 100$ eV); we therefore call
these ``classical'' supersoft sources, or simply supersoft sources (SSSs).
Eighteen VSSs may either have small (< 10%) hard components, or slightly higher
effective temperatures (but still < 350 eV). We refer to these VSSs as
quasisoft sources (QSSs). While hot white dwarf models may apply to SSSs, the
effective temperatures of QSSs are too high, unless, e.g., the radiation
emanates from only a small portion of surface. Two of the SSSs were first
detected and identified as such through ROSAT observations. One SSS and one QSS
may be identified with symbiotics, and 2 SSSs with supernova remnants. Both
SSSs and QSSs in the disk are found near star-forming regions, possibly
indicating that they are young. VSSs in the outer disk and halo are likely to
be old systems; in these regions, there are more QSSs than SSSs, which is
opposite to what is found in fields closer to the galaxy center. The largest
density of bright VSSs is in the bulge; some of the bulge sources are close
enough to the nucleus to be remnants of the tidal disruption of a giant by the
massive central black hole. By using Chandra data in combination with ROSAT and
XMM observations, we find most VSSs to be highly variable, fading from or
brightening toward detectability on time scales of months. There is evidence
for VSSs with low luminosities ($\sim 10^{36}$ erg s$^{-1}$).

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The categories of this paper are: math-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: math
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The categories of this paper are: econ
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The categories of this paper are: supr-con
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The categories of this paper are: atom-ph
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The categories of this paper are: q-fin
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The categories of this paper are: alg-geom
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The categories of this paper are: stat
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The categories of this paper are: quant-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-th
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The categories of this paper are: adap-org
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The categories of this paper are: q-bio