Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence of a decrease of kHz QPO peak separation towards low
  frequencies in 4U 1728-34 (GX 354-0)   We have produced the colour-colour diagram of all the observations of 4U
1728-34 available in the Rossi X-ray Timing Explorer public archive (from 1996
to 2002) and found observations filling in a previously reported 'gap' between
the island and the banana X-ray states. We have made timing analysis of these
gap observations and found, in one observation, two simultaneous kHz
quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit
in the overall trend of the source. The 'lower' kHz QPO has a centroid
frequency of ~308 Hz. This is the lowest 'lower' kHz QPO frequency ever
observed in 4U 1728-34. The peak frequency separation between the 'upper' and
the 'lower' kHz QPO is 274+/-11 Hz, significantly smaller than the constant
value of ~350 Hz found when the 'lower' kHz QPO frequency is between ~500 and
800 Hz. This is the first indication in this source for a significant decrease
of kHz QPO peak separation towards low frequencies. We briefly compare the
result to theoretical models for kHz QPO production.

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The categories of this paper are: funct-an
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The categories of this paper are: adap-org
3
The categories of this paper are: physics
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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: plasm-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: eess
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The categories of this paper are: math-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-th
1
The categories of this paper are: cs
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The categories of this paper are: dg-ga
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The categories of this paper are: patt-sol