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

## TL;DR

This study demonstrates that radial ionisation profiles in accretion discs can produce steep X-ray reflection emissivity profiles in AGN, affecting black hole spin measurements and emphasizing the importance of considering ionisation structure.

## Contribution

The paper provides a detailed analysis of how radially-structured disc ionisation influences X-ray reflection spectra, highlighting its impact on emissivity profiles and black hole spin estimates.

## Key findings

- Radial ionisation profiles can produce steep emissivity profiles.
- High inner disc ionisation affects spin measurement accuracy.
- Radial ionisation effects can surpass geometrical influences.

## 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 ionisation. Svoboda et al. (2012) showed on a single example that assuming a radially-structured ionisation 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 micro-calorimeter 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 ionisation: 1) a radial profile for the disc ionisation, 2) a constant disc ionisation. We found that steep emissivity profiles can be indeed achieved due to the radial profile of the disc ionisation, which becomes more important for the cases where the corona is located at low heights above the black hole and this effect may be even more prominent than the geometrical effects. We also found that the cases with high inner disc ionisation, rapidly decreasing with radius, may result in an inaccurate black hole spin measurements.