Impact of ionization and electron density gradients in X-ray reflection spectroscopy measurements

Abstract

The models currently used for the analysis of the reflection spectra of black holes usually assume a disk with constant ionization and electron density. However, there is some debate on the impact of these assumptions on the estimate of the properties of the sources, in particular when the fits suggest very steep emissivity profiles in the inner part of the accretion disk. In this work, we re-analyze a selected set of high-quality NuSTAR and Suzaku data of Galactic black holes and we fit the reflection component with three different models: relxill_nk, in which the ionization parameter and the electron density are constant, relxillion_nk, where the electron density is still constant but the ionization profile is described by a power law, and relxilldgrad_nk, where the electron density profile is described by a power law and the ionization profile is calculated self-consistently from the electron density and the emissivity. While relxillion_nk may provide slightly better fits, we do not find any substantial difference in the estimate of the properties of the sources among the three models. Our conclusion is that models with constant electron density and ionization parameter are probably sufficient, in most cases, to fit the currently available X-ray data of accreting black holes.

Testing Relativistic Reflection Models with GRMHD Simulations of Accreting Black Holes

“…Some approximations have been removed in more recent reflection models and tested with observations. For instance, there are now models that permit nontrivial radial disk profiles of the ionization parameter and of the electron density (Abdikamalov et al 2021a(Abdikamalov et al , 2021b, but such improvements do not seem to be strictly necessary for current spin measurements (Mall et al 2022). Specific coronal geometries have been investigated in a few studies, but the general conclusion is that phenomenological profiles like a broken power law or a twice broken power law can fit well the current spectra without introducing undesired bias in the parameter estimates (see, e.g., Wilkins & Fabian 2011, 2012Gonzalez et al 2017;Riaz et al 2022a).…”

Testing relativistic reflection models with GRMHD simulations of accreting black holes

“…Some approximations have been removed in more recent reflection models and tested with observations. For instance, there are now models that permit a nontrivial radial disk profile of the ionization parameter and of the electron density (Abdikamalov et al 2021b,a), but such improvements do not seem to be strictly necessary for current spin measurements (Mall et al 2022). Specific coronal geometries have been investigated in a few studies, but the general conclusion is that phenomenological profiles like a broken power law or a twice broken power law can fit well the current spectra without introducing undesired bias in the parameter estimates (see, e.g., Wilkins & Fabian 2011, 2012Gonzalez et al 2017;Riaz et al 2022a).…”