Funnel wall jets and the nature of the soft X-ray excess

TL;DR

The paper investigates the origin of the soft X-ray excess in AGN, suggesting it may be due to high-velocity, possibly clumpy, magnetically driven outflows rather than continuous winds driven by radiation or thermal processes.

Contribution

It provides an analysis of the velocity requirements for absorption models of the soft X-ray excess and discusses the plausibility of different wind-driving mechanisms.

Findings

High outflow velocities (~0.9c) are needed to explain the soft X-ray excess.

Radiation and thermal driving are unlikely mechanisms for such high-velocity winds.

Clumpy or partial covering material is a more plausible explanation for the soft excess.

Abstract

The smooth soft X-ray excess seen in many type-1 AGN can be well described by models of absorption in partially ionized material with a large velocity dispersion, often physically interpreted as a radiatively driven accretion disk wind. However, the state-of-the-art XSCORT code, which calculates the photoionized radiative transfer through a differentially outflowing absorber, shows that terminal velocities of order ~0.9c are required in order to reproduce the soft X-ray excess. Such a high outflow velocity rules out UV-line driving, continuum radiation driving, and thermal driving as mechanisms for producing the wind. Entrainment of material by the magnetically driven jet is the only plausible origin of such a high velocity flow, but numerical simulations of jets and associated outflows do not currently show sufficient material at high enough velocities to reproduce the soft X-ray excess. If the soft excess is produced by absorption then it seems more likely that the material is clumpy and/or only partially covers the source rather than forming a continuous outflow.