Abstract
We present a simplified model of Active Galactic Nucleus (AGN) continuum
emission designed for photoionization modeling. The new model oxaf
reproduces the diversity of spectral shapes that arise in physically-based
models. We identify and explain degeneracies in the effects of AGN parameters
on model spectral shapes, with a focus on the complete degeneracy between the
black hole mass and AGN luminosity. Our re-parametrized model oxaf
removes these degeneracies and accepts three parameters which directly describe
the output spectral shape: the energy of the peak of the accretion disk
emission $E_peak$, the photon power-law index of the non-thermal
emission $\Gamma$, and the proportion of the total flux which is emitted in the
non-thermal component $p_NT$. The parameter $E_peak$ is
presented as a function of the black hole mass, AGN luminosity, and `coronal
radius' of the optxagnf model upon which oxaf is based. We show
that the soft X-ray excess does not significantly affect photoionization
modeling predictions of strong emission lines in Seyfert narrow-line regions.
Despite its simplicity, oxaf accounts for opacity effects where the
accretion disk is ionized because it inherits the `color correction' of \sc
optxagnf. We use a grid of mappings photoionization models with \sc
oxaf ionizing spectra to demonstrate how predicted emission-line ratios on
standard optical diagnostic diagrams are sensitive to each of the three \sc
oxaf parameters. The oxaf code is publicly available in the Astrophysics
Source Code Library.
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