Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Ghost in the Machine   I examine simple tests for the presence of accretion disks in AGN - changes
of surface brightness with viewing angle, changes of colour temperature with
luminosity, and behaviour during variability. AGN observations pass the first
two tests but fail the third, unless there is some previously unobserved source
of heating - the ``ghost in the machine''.

34
The categories of this paper are: chem-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: solv-int
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The categories of this paper are: hep-ex
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The categories of this paper are: stat
11
The categories of this paper are: hep-lat
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The categories of this paper are: cmp-lg
7
The categories of this paper are: hep-th
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The categories of this paper are: acc-phys
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The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
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The categories of this paper are: adap-org
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The categories of this paper are: eess
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom