Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Softening of the Metagalactic Ionizing Background from Internal He II
  Absorption in Quasars   Quasars and active galactic nuclei (AGN) are significant contributors to the
metagalactic ionizing background at redshifts z < 3. Recent HST/COS composite
spectra of AGN find a harder flux distribution in the Lyman continuum, F_nu ~
nu^{-alpha_s} (alpha_s = 1.41+/-0.15) compared to previous studies. This index
appears to be inconsistent with observed He II / H I absorption ratios (eta) in
the Lya forest. We explore effects of internal AGN absorption in the He II (4
ryd) continuum using an analytic source-function model of the ionizing
background in which the emissivity (j_nu) arises from quasars, reprocessed by
the opacity (kappa_nu) of the intervening Lya forest and distinct AGN escape
fractions f_esc(HI) and f_esc(HeII) at 1~ryd and 4~ryd, respectively. We also
examine H I and He II photoelectric heating from the reprocessed background,
whose spectral index (alpha_b > alpha_s) depends on alpha_s, the H I column
density slope beta, and the ratio R = f_esc(HI) / f_esc(HeII). We compare the
model to Lya absorption lines of He II and H I toward the quasar HE~2347-4342.
Internal AGN absorption with f_esc(HeII) = 0.6-0.8 but f_esc(HI) = 1 would
increase the index by Delta-alpha_b = 0.3-1.0, corresponding to eta = 60-200
for beta = 1.5-1.6, in agreement with HST/COS observations at z = 2.5-2.9. The
observed range of ratios, eta < 200, constrains alpha_b < 3.4 and f_esc(HeII) >
0.4. Individual AGN with softer spectra, alpha_s > 1.7, and more internal He II
absorption could produce a few absorbers with eta > 300 in proximity to AGN
transverse to the sight line.

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The categories of this paper are: math-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ex
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The categories of this paper are: adap-org
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The categories of this paper are: ao-sci
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The categories of this paper are: alg-geom
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The categories of this paper are: bayes-an
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The categories of this paper are: mtrl-th
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-th
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The categories of this paper are: dg-ga
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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: quant-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: acc-phys