Interaction of the accretion flows in corona and disk near the black hole in AGN

TL;DR

This paper investigates the interaction between corona and disk in AGN accretion flows, comparing theoretical condensation models with observational data to understand the coexistence and spectral features of these flows.

Contribution

It introduces observational validation of the condensation model for corona-disk interaction in AGN using simultaneous X-ray and UV-optical data.

Findings

Broad iron Kα lines are frequently detected in AGN spectra.

Spectral properties depend on the Eddington ratio, consistent with the model.

Coronal emission is concentrated in a narrow inner region.

Abstract

Accretion flows toward black holes can be of a quite different nature, described as an optically thick cool gas flow in a disk for high accretion rates or as a hot coronal optically thin gas flow for low accretion rates, possibly affected by outflowing gas. The detection of broad iron emission lines in active galactic nuclei (AGN) indicates the coexistence of corona and disk. The appearance and relative strength of such flows essentially depends on their interaction. Liu et al. suggested that condensation of gas from the corona to the disk allows to understand accretion flows of comparable strength of emission. Matter inflow due to gravitational capture of gas is important for the condensation process. We discuss observational features predicted by the model. Data from simultaneous observations of AGN with {\it {Swift's}} X-ray and UV-optical telescopes are compared with the theoretical predictions. The frequent detection of broad iron K$\alpha$ emission lines and the dependence of the emitted spectra on the Eddington ratio, described by the values of the photon index $\Gamma$ and the two-point spectral index $\alpha_{\rm{ox}}$ are in approximate agreement with the predictions of the condensation model; the latter, however, with a large scatter. The model further yields a coronal emission concentrated in a narrow inner region as is also deduced from the analysis of emissivity profiles. The accretion flows in bright AGN could be described by the accretion of stellar wind or interstellar medium and its condensation into a thin disk.