Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 4U 1957+11: a persistent low-mass X-ray binary and black-hole candidate
  in the high state?   We report on several pointed Rossi X-ray Timing Explorer observations of the
enigmatic low-mass X-ray binary (LMXB) 4U 1957+11 at different X-ray
luminosities. The luminosity of the source varied by more than a factor of four
on time scales of months to years. The spectrum of the source tends to get
harder when its luminosity increases. Only very weak (1%-2% rms amplitude;
0.001-10 Hz; 2-60 keV) rapid X-ray variability was observed during the
observations. A comparison of the spectral and temporal behaviour of 4U1957+11
with other X-ray binary systems, in particular LMC X-3, indicates that 4U
1957+11 is likely to be a persistent LMXB harboring a black hole and which is
persistently in the black-hole high state. If confirmed, it would be the only
such system known.

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The categories of this paper are: hep-lat
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The categories of this paper are: q-bio
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The categories of this paper are: hep-th
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The categories of this paper are: nlin
21
The categories of this paper are: adap-org
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The categories of this paper are: gr-qc
0
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: funct-an
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The categories of this paper are: math
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The categories of this paper are: stat
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The categories of this paper are: comp-gas
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The categories of this paper are: physics
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The categories of this paper are: nucl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: quant-ph