UV line-driven disc wind as the origin of UltraFast Outflows in AGN

Abstract

ABSTRACT UltraFast Outflows (UFOs) are observed in some active galactic nuclei (AGN), with blueshifted and highly ionized Fe-K absorption features. AGN typically have an ultraviolet (UV-) bright accretion flow, so UV line driving is an obvious candidate for launching these winds. However this mechanism requires material with UV opacity, in apparent conflict with the observed high-ionization state of the wind. In this paper, we synthesize the X-ray energy spectra resulting from different lines of sight through a state of the art radiation hydrodynamics UV line-driven disc wind simulation. We demonstrate that there are some lines of sight that only intercept highly ionized and fast outflowing material. The cooler material required for the UV line driving acceleration is out of the line of sight, close to the disc, shielded from the X-rays by a failed wind. We fit these simulated wind spectra to data from the archetypal UFO source PG 1211+143 and show that they broadly reproduce the depth and velocity of the iron absorption lines seen. This directly demonstrates that UV line driving is a viable mechanism to launch even the fastest UFOs. We simulate microcalorimeter observations of this wind and show that their high-energy resolution can resolve the detailed structure in the wind and recover the wind energetics when combined with models that correctly estimate the line formation radius of the wind. New data from microcalorimeters will pave the way for physical predictions of AGN wind feedback in cosmological simulations.