New Spectral Model for Constraining Torus Covering Factors from
  Broadband X-ray Spectra of Active Galactic Nuclei
  The basic unified model of active galactic nuclei (AGN) invokes an
anisotropic obscuring structure, usually referred to as a torus, to explain AGN
obscuration as an angle-dependent effect. We present a new grid of X-ray
spectral templates based on radiative transfer calculations in neutral gas in
an approximately toroidal geometry, appropriate for CCD-resolution X-ray
spectra (FWHM > 130 eV). Fitting the templates to broadband X-ray spectra of
AGN provides constraints on two important geometrical parameters of the gas
distribution around the supermassive black hole: the average column density and
the covering factor. Compared to the currently available spectral templates,
our model is more flexible, and capable of providing constraints on the main
torus parameters in a wider range of AGN. We demonstrate the application of
this model using hard X-ray spectra from NuSTAR (3-79 keV) for four AGN
covering a variety of classifications: 3C 390.3, NGC 2110, IC 5063, and NGC
7582. This small set of examples was chosen to illustrate the range of possible
torus configurations, from disk-like to sphere-like geometries with column
densities below, as well as above, the Compton-thick threshold. This diversity
of torus properties challenges the simple assumption of a standard
geometrically and optically thick toroidal structure commonly invoked in the
basic form of the unified model of AGN. Finding broad consistency between the
our constraints and those from infrared modeling, we discuss how the approach
from the X-ray band complements similar measurements of AGN structures at other
wavelengths.
