Modeling high-resolution spectra from x-ray illuminated accretion disks.

摘要

This work focuses on the study of X-ray illuminated accretion disks around black holes by modeling their structure and reprocessed emission. The calculation of new models for the reflected spectra consider the effects of incident X-rays on the surface of an accretion disk by solving simultaneously the equations of radiative transfer, energy balance and ionization equilibrium over a large range of column densities. Plane-parallel geometry and azimuthal symmetry are assumed, such that each calculation corresponds to a ring at a given distance from the central object. 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 isonuclear sequences of iron, oxygen, and nitrogen. The redistribution of photons due to Compton scattering 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 partial photoionization cross sections are computed with a portfolio of publicly available atomic physics codes: AUTOSTRUCTURE, HFR, and BPRM..;The shape of the Fe K-line is perhaps one of the most important features in the Xray spectrum of accreting sources. Therefore, the effect of fluorescent Kα line emission and absorption in the emitted spectrum is explored, as well as the dependence of the spectrum on the strength of the incident X-rays and other input parameters and the importance of Comptonization on the emitted spectrum. These calculations predict under which conditions the 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, these models suggest that the energy redistribution of the photons due to Compton scattering also affects the line profile and it is responsible for an important fraction of the broadening.