Modeling High-Velocity QSO Absorbers with Photoionized MHD Disk-Winds

TL;DR

This paper models magnetohydrodynamic accretion disk winds in QSOs, showing they can produce high-velocity ionized absorbers consistent with observations, especially at low X-ray-to-UV flux ratios.

Contribution

It introduces a new MHD wind model for QSOs that explains high-velocity absorbers based on the spectral index, extending previous models from Seyfert galaxies.

Findings

High-velocity ionized absorbers are produced in MHD winds with low $ m ext{ox}$.

Absorber velocities increase as $ m ext{ox}$ decreases.

Model predictions align with observed velocities and column densities.

Abstract

We extend our modeling of the ionization structure of magnetohydrodynamic (MHD) accretion-disk winds, previously applied to Seyfert galaxies, to a population of quasi-stellar-objects (QSOs) of much lower X-ray-to-UV flux ratios, i.e. smaller $\alpha_{\rm ox}$ index, motivated by UV/X-ray ionized absorbers with extremely high outflow velocities in UV-luminous QSOs. We demonstrate that magnetically-driven winds ionized by a spectrum with $\alpha_{\rm ox} \simeq -2$ can produce the charge states responsible for \civ ~and \fexxv/\fexxvi ~absorption in wind regions with corresponding maximum velocities of $v$(\civ) $\lsim 0.1c$ and $v({\rm \fexxv}) \lsim 0.6 c$ (where $c$ is the speed of light) and column densities $N_H \sim 10^{23}-10^{24}$ cm$^{-2}$, in general agreement with observations. In contrast to the conventional radiation-driven wind models, {\it high-velocity flows are always present in our MHD-driven winds} but manifest in the absorption spectra only for $\alpha_{\rm ox} \lsim -2$, as larger $\alpha_{\rm ox}$ values ionize the wind completely out to radii too large to demonstrate the presence of these high velocities. We thus predict increasing velocities of these ionized absorbers with decreasing (steeper) $\alpha_{\rm ox}$, a quantity that emerges as the defining parameter in the kinematics of the AGN UV/X-ray absorbers.