Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Accurate positions for the ULXs NGC 7319-X4 and NGC 5474-X1 and limiting
  magnitudes for their optical counterparts   In this paper we report accurate Chandra positions for two ultraluminous
X-ray sources: NGC 7319-X4 at Right Ascension (RA) = 339.02917(2) deg,
Declination (Dec) = 33.97476(2) deg and NGC 5474-X1 at RA = 211.24859(3) deg,
Dec = 53.63584(3) deg. We perform bore-sight corrections on the Chandra X-ray
Satellite observations of these sources to get to these accurate positions of
the X-ray sources and match these positions with archival optical data from the
Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. We do
not find the optical counterparts: the limiting absolute magnitudes of the
observations in the WFPC2 standard magnitude system are B = -7.9, V = -8.7 and
I = -9.3 for NGC 7319-X4 and U = -6.4 for NGC 5474-X1. We report on the X-ray
spectral properties and we find evidence for X-ray variability in NGC 5474-X1.
Finally, we briefly discuss several options for the nature of these ULXs.

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The categories of this paper are: quant-ph
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The categories of this paper are: physics
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The categories of this paper are: atom-ph
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The categories of this paper are: adap-org
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-th
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The categories of this paper are: econ
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The categories of this paper are: q-bio
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The categories of this paper are: cs
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The categories of this paper are: nucl-ex
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-ex