Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The fading of Cassiopeia A, and improved models for the absolute
  spectrum of primary radio calibration sources   Based on five years of observations with the 40-foot telescope at Green Bank
Observatory (GBO), Reichart & Stephens (2000) found that the radio source
Cassiopeia A had either faded more slowly between the mid-1970s and late 1990s
than Baars et al. (1977) had found it to be fading between the late 1940s and
mid-1970s, or that it had rebrightened and then resumed fading sometime between
the mid-1970s and mid-1990s, in L band (1.4 GHz). Here, we present 15
additional years of observations of Cas A and Cyg A with the 40-foot in L band,
and three and a half additional years of observations of Cas A, Cyg A, Tau A,
and Vir A with GBO's recently refurbished 20-meter telescope in L and X (9 GHz)
bands. We also present a more sophisticated analysis of the 40-foot data, and a
reanalysis of the Baars et al. (1977) data, which reveals small, but
non-negligible differences. We find that overall, between the late 1950s and
late 2010s, Cas A faded at an average rate of $0.670 \pm 0.019$ %/yr in L band,
consistent with Reichart & Stephens (2000). However, we also find, at the
6.3$\sigma$ credible level, that it did not fade at a constant rate. Rather,
Cas A faded at a faster rate through at least the late 1960s, rebrightened (or
at least faded at a much slower rate), and then resumed fading at a similarly
fast rate by, at most, the late 1990s. Given these differences from the
original Baars et al. (1977) analysis, and given the importance of their fitted
spectral and temporal models for flux-density calibration in radio astronomy,
we update and improve on these models for all four of these radio sources. In
doing so, we additionally find that Tau A is fading at a rate of
$0.102^{+0.042}_{-0.043}$ %/yr in L band.

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The categories of this paper are: physics
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The categories of this paper are: math-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: ao-sci
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The categories of this paper are: q-fin
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The categories of this paper are: econ
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The categories of this paper are: patt-sol
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The categories of this paper are: nlin
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The categories of this paper are: nucl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: acc-phys
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The categories of this paper are: funct-an
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The categories of this paper are: math
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The categories of this paper are: bayes-an
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The categories of this paper are: alg-geom
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The categories of this paper are: q-alg