Radiation Hydrodynamic Simulations of Line-Driven Disk Winds for Ultra Fast Outflows

TL;DR

This study uses radiation hydrodynamic simulations to demonstrate that line-driven disk winds can produce ultra fast outflows in luminous active galactic nuclei, matching observed properties and potentially explaining their prevalence.

Contribution

The paper provides the first detailed 2D simulations showing how spectral line forces generate UFOs across a range of black hole masses and Eddington ratios, linking theory with observations.

Findings

Line-driven winds produce UFO-like outflows with velocities ~10% of light speed.

UFOs are statistically observable in 13-28% of luminous AGNs.

Results are insensitive to X-ray luminosity and disk surface density.

Abstract

Using two-dimensional radiation hydrodynamic simulations, we investigate origin of the ultra fast outflows (UFOs) that are often observed in luminous active galactic nuclei (AGNs). We found that the radiation force due to the spectral lines generates strong winds (line-driven disk winds) that are launched from the inner region of accretion disks (~30 Schwarzschild radii). A wide range of black hole masses ($M_{\rm BH}$) and Eddington ratios ($\varepsilon$) was investigated to study conditions for causing the line-driven winds. For $M_{\rm BH} = 10^6-10^9 M_\odot$ and $\varepsilon = 0.1-0.7$, funnel-shaped disk winds appear, in which dense matter is accelerated outward with an opening angle of 70-80 deg and with 10% of the light speed. If we observe the wind along its direction, the velocity, the column density, and the ionization state are consistent with those of the observed UFOs. As long as the obscuration by the torus does not affect the observations of X-ray bands, the UFOs could be statistically observed in about 13-28% of the luminous AGNs, which is not inconsistent with the observed ratio (~40%). We also found that the results are insensitive to the X-ray luminosity and the density of the disk surface. Thus, we can conclude that the UFOs could exist in any luminous AGNs, such as narrow-line Seyfert 1s (NLS1s) and quasars with $\varepsilon > 0.1$, in which fast line-driven winds are associated.