Modeling High-Resolution Spectra from X-ray Illuminated Accretion Disks
This work focuses on the study of X-ray illuminated accretion disks aroundblack holes by modeling their structure and reprocessed emission. The calculationof new models for the reflected spectra consider the effects of incident X-rays on thesurface of an accretion disk by solving simultaneously the equations of radiativetransfer, energy balance and ionization equilibrium over a large range ofcolumn densities. Plane-parallel geometry and azimuthal symmetry are assumed,such that each calculation corresponds to a ring at a given distance from the centralobject. The radiation transfer equations are solved by using the Feautrier scheme.Ionization and thermal balance are solved by using the photoionization code XSTAR,including the most recent and complete atomic data for K-shell of the isonuclearsequences of iron, oxygen, and nitrogen. The redistribution of photons due to Comptonscattering is included using a Gaussian approximation for the Compton kernel.The atomic data for nitrogen ions, namely, level energies, wavelengths,gf-values, radiative widths, total and partial Auger widths, and total and partialphotoionization cross sections are computed with a portfolio of publicly availableatomic physics codes: AUTOSTRUCTURE, HFR, and BPRM.The shape of the Fe K-line is perhaps one of the most important features inthe X-ray spectrum of accreting sources. Therefore, the effect of fluorescentK&alpha line emission and absorption in the emitted spectrumis explored, as well as the dependence of the spectrum on the strength of the incidentX-rays and other input parameters and the importance of Comptonization on the emittedspectrum. These calculations predict under which conditionsthe line is formed, providing information about the ionization stage of the emitting gas.The width of this line is often related to relativistic effects (i.e. gravitational redshift),since the emitting gas may be located in regions close to the black hole. However, thesemodels suggest that the energy redistribution of the photons due to Compton scatteringalso affects the line profile and it is responsible for an important fraction of thebroadening.
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