Multiphase AGN winds from X-ray irradiated disk atmospheres

TL;DR

This paper investigates how thermal instability influences the clumpiness and structure of X-ray irradiated disk winds in active galactic nuclei, revealing new dynamics and observational signatures.

Contribution

It identifies the role of the Bernoulli function in thermal instability growth and demonstrates how buoyancy-driven bubbles cause clumpiness in disk winds through numerical simulations.

Findings

Clumpiness arises from buoyancy disrupting stratification.

Hot bubbles from thermal instability fragment the atmosphere.

Observable X-ray absorption features are affected by this clumpiness.

Abstract

The mechanism of thermal driving for launching mass outflows is interconnected with classical thermal instability (TI). In a recent paper, we demonstrated that as a result of this interconnectedness, radial wind solutions of X-ray heated flows are prone to becoming clumpy. In this paper, we first show that the Bernoulli function determines whether or not the entropy mode can grow due to TI in dynamical flows. Based on this finding, we identify a critical `unbound' radius beyond which TI should accompany thermal driving. Our numerical disk wind simulations support this result and reveal that clumpiness is a consequence of buoyancy disrupting the stratified structure of steady state solutions. Namely, instead of a smooth transition layer separating the highly ionized disk wind from the cold phase atmosphere below, hot bubbles formed from TI rise up and fragment the atmosphere. These bubbles first appear within large scale vortices that form below the transition layer, and they result in the episodic production of distinctive cold phase structures referred to as irradiated atmospheric fragments (IAFs). Upon interacting with the wind, IAFs advect outward and develop extended crests. The subsequent disintegration of the IAFs takes place within a turbulent wake that reaches high elevations above the disk. We show that this dynamics has the following observational implications: dips in the absorption measure distribution are no longer expected within TI zones and there can be a less sudden desaturation of X-ray absorption lines such as \OVIII as well as multiple absorption troughs in \FeXXVK.