Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: RXTE Studies of Long-Term X-ray Spectral Variations in 4U 1820-30   We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1820-30 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996-7. 4U 1820-30 is an ``atoll'' source X-ray burster
(XRB) located in the globular cluster NGC 6624. It is known to have an 11
minute binary period and a ~176 day modulation in its 2--12 keV flux.
Observations were made with the PCA and HEXTE instruments on RXTE at roughly
one-month intervals to sample this long-term period and study flux-related
spectral changes. There are clear correlations between our fitted spectral
parameters and both the broad-band (2--50 keV) flux and the position in the
color-color diagram, as described by the parameter S_a introduced by Mendez et
al. (1999). In addition, we find a strong correlation between the position in
the color-color diagram and the frequencies of the kilohertz quasi-periodic
oscillations (kHz QPOs) reported by Zhang et al. (1998). This lends further
support to the notion that evidence for the last stable orbit in the accretion
disk of 4U 1820-30 has been observed. For a model consisting of Comptonization
of cool photons by hot electrons plus an additional blackbody component, we
report an abrupt change in the spectral parameters at the same accretion rate
at which the kHz QPOs disappear. For a model consisting of a multicolor disk
blackbody plus a cut-off power law, we find that the inner disk radius reaches
a minimum at the same accretion rate at which the kHz QPO frequency saturates,
as expected if the disk reaches the last stable orbit. Both models face
theoretical and observational problems when interpreted physically for this
system.

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The categories of this paper are: funct-an
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The categories of this paper are: nlin
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The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga
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The categories of this paper are: q-fin
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The categories of this paper are: hep-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: econ
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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: physics
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The categories of this paper are: eess
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The categories of this paper are: q-alg
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The categories of this paper are: cs
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The categories of this paper are: mtrl-th