Steep X-ray reflection emissivity profiles in AGN as the result of radially structured disc ionization

TLDR: In this paper, the authors showed that steep emissivity profiles can be achieved due to the radial profile of the disc ionization, which becomes more important for the cases where the corona is located at low heights above the black hole and this effect may even be more prominent than the geometrical effects.

Abstract: X-ray observations suggest high compactness of coronae in active galactic nuclei as well as in X-ray binaries. The compactness of the source implies a strong radial dependence in the illumination of the accretion disc. This will, for any reasonable radial profile of the density, lead to a radial profile of the disc ionization. Svoboda et al. showed on a single example that assuming a radially structured ionization profile of the disc can cause an artificial increase of the radial emissivity parameter. We further investigate how the X-ray spectra are modified and quantify this effect for a wide range of parameters. Computations are carried out with the current state-of-the-art models for relativistic reflection. We simulated spectra using the response files of the microcalorimeter X-IFU, which is planned to be on board of Athena. We assumed typical parameters for X-ray bright Seyfert-1 galaxies and considered two scenarios for the disc ionization: (1) a radial profile for the disc ionization and (2) a constant disc ionization. We found that steep emissivity profiles can be indeed achieved due to the radial profile of the disc ionization, which becomes more important for the cases where the corona is located at low heights above the black hole and this effect may even be more prominent than the geometrical effects. We also found that the cases with high inner disc ionization, rapidly decreasing with radius, may result in an inaccurate black hole spin measurements.