Synthetic X-ray spectra for simulations of the dynamics of an accretion flow irradiated by a quasar

TL;DR

This study uses radiative transfer calculations on hydrodynamical simulations to predict X-ray spectra of outflows in active galactic nuclei, aiding the interpretation of observations and guiding future models.

Contribution

It introduces a detailed method combining hydrodynamical simulations with radiative transfer to analyze X-ray spectra of AGN outflows, highlighting the effects of inflow and outflow structures.

Findings

Outflows cause strong absorption at angles < 35°.

Infalling gas does not produce strong absorption features.

Predicted emission features arise from outflow scattering and fluorescence.

Abstract

Ultraviolet and X-ray observations show evidence of outflowing gas around many active galactic nuclei. Some of these outflows may be driven off gas infalling towards the central black hole. We perform radiative transfer calculations to compute the gas ionization state and X-ray spectra for two- and three-dimensional (3D) hydrodynamical simulations of this outflow-from-inflow scenario. By comparison with observations, our results can be used to test the theoretical models and guide future numerical simulations. We predict both absorption and emission features, most of which are formed in a polar funnel of outflowing gas. This outflow causes strong absorption for observer orientation angles of < 35 degrees. Particularly in 3D, the strength of this absorption varies significantly for different lines-of-sight owing to the fragmentary structure of the gas flow. Although infalling material occupies a large fraction of the simulation volume, we do not find that it imprints strong absorption features since the ionization state is very high. Thus, an absence of observed inflow absorption features does not exclude the models. The main spectroscopic consequence of the infalling gas is a scattered continuum component that partially re-fills the absorption features caused by the outflowing polar funnel. Fluorescence and scattering in the outflow is predicted to give rise to several emission features for all observer orientations. For the hydrodynamical simulations considered we find both ionization states and column densities for the outflowing gas that are too high to be quantitatively consistent with well-observed X-ray absorption systems. Nevertheless, our results are qualitatively encouraging and further exploration of the model parameter space is warranted. (Abridged.)