Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A Systematic Study of Variability in a Sample of Ultraluminous X-ray
  Sources   We present results from a study of short term variability in 19 archival
observations by XMM-Newton of 16 Ultraluminous X-ray Sources (ULXs). Eight
observations (six sources) showed intrinsic variability with power spectra in
the form of either a power law or broken power law-like continuum and in some
cases quasi-periodic oscillations (QPOs). The remaining observations were used
to place upper limits on the strength of possible variability hidden within.
Seven observations (seven sources) yielded upper limits comparable to, or
higher than, the values measured from those observations with detectable
variations. These represented the seven faintest sources all with f_x <
3x10^-12 erg cm^-2 s^-1. In contrast there are four observations (three
sources) that gave upper limits significantly lower than both the values
measured from the ULX observations with detectable variations, and the values
expected by comparison with luminous Galactic black hole X-ray binaries (BHBs)
and Active Galactic Nuclei (AGN) in the observed frequency bandpass (10^-3 - 1
Hz). This is the case irrespective of whether one assumes characteristic
frequencies appropriate for a stellar mass (10 M_sun) or an intermediate mass
(1000 M_sun) black hole, and means that in some ULXs the variability is
significantly suppressed compared to bright BHBs and AGN. We discuss ways to
account for this unusual suppression in terms of both observational and
intrinsic effects and whether these solutions are supported by our results.

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The categories of this paper are: comp-gas
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The categories of this paper are: math
31
The categories of this paper are: ao-sci
5
The categories of this paper are: gr-qc
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The categories of this paper are: math-ph
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The categories of this paper are: supr-con
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The categories of this paper are: adap-org
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The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-th
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The categories of this paper are: q-alg
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The categories of this paper are: atom-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: funct-an