Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: What the Optical Recombination Lines Can Tell Us About the Broad Line
  Regions of Active Galactic Nuclei   We investigate the effect of a global change in the ionizing continuum level
on the behavior of the strong optical broad emission-lines seen in spectra of
the nuclear emission-line regions of active galactic nuclei (AGN), including
the Balmer lines, HeI 5876, and HeII 4686. Unlike most of the prominent heavy
element lines found in the UV, the optical hydrogen and helium recombination
lines' emissivities are strongly dependent upon the incident continuum flux,
since these lines arise out of excited states whose optical depths depend upon
the incident flux of photons. Using photoionization calculations we determine
the luminosity-dependent responsivities of these lines for a general model of
the broad emission line region (BLR), with the purpose of establishing them as
important probes of the physical conditions within the BLR of AGNs. The
dependence of these lines' emissivities on the incident photon flux invokes a
dependence in their responsivities on distance from the central continuum
source. In particular, the responsivities of these lines are generally
anti-correlated with the incident photon flux. Thus, their responsivities vary
with distance within the BLR for a fixed continuum luminosity and change with
time as the continuum source varies. Consequently, after correcting for light
travel time effects the response of the Balmer and optical helium lines should
generally be strongest during low continuum luminosity states. Responsivity
that depends upon photon flux and continuum state may explain a number of
outstanding problems currently under investigation in broad line variability
studies of these and other emission lines. [Abridged]

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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: supr-con
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The categories of this paper are: cmp-lg
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The categories of this paper are: funct-an
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The categories of this paper are: hep-th
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The categories of this paper are: cs
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The categories of this paper are: q-fin
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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: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: plasm-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: stat
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The categories of this paper are: chao-dyn