Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Imaging Sources in the Third Realization of the International Celestial
  Reference Frame   The third iteration of the International Celestial Reference Frame (ICRF3) is
made up of 4536 quasars observed at S/X bands using Very Long baseline
Interferometry (VLBI). These sources are high redshift quasars, typically
between $1<z<2$, that are believed to host active galactic nuclei (AGN) at
their centers. The position of compact radio sources can be determined better
than sources with large amounts of extended radio structure. Here we report
information on a series of 20 observations from January 2017 through December
2017 which were designed for precise astrometry and to monitor the structure of
sources included in the ICRF3. We targeted 3627 sources over the one year
campaign and found the median flux density of 2697 detected sources at S-band
is 0.13 Jy, and the flux density of 3209 sources detected at X-band is 0.09 Jy.
We find that $70\%$ of detected sources in our campaign are considered compact
at X-band and ideal for use in the ICRF and $89\%$ of the 2615 sources detected
at both frequencies have a flat spectral index, $\alpha>0.5$

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The categories of this paper are: stat
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The categories of this paper are: dg-ga
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The categories of this paper are: econ
10
The categories of this paper are: q-bio
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The categories of this paper are: solv-int
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The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: bayes-an
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The categories of this paper are: eess
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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: atom-ph