The impact of accretion disk winds on the X-ray spectrum of AGN: Part 1 - XSCORT

TL;DR

This paper introduces XSCORT, a numerical radiative transfer code that models AGN X-ray spectra through accretion disk winds, revealing complex spectral features and challenging the idea that winds cause the soft X-ray excess.

Contribution

The paper presents a self-consistent spectral modeling method for AGN winds, improving upon ad-hoc models and exploring the impact of wind parameters on X-ray spectra.

Findings

Spectral features are highly sensitive to wind velocity and density.

Fast winds with velocities ~c can smooth out spectral features.

Accretion disk winds are unlikely to produce the soft X-ray excess.

Abstract

(abridged) The accretion disk in AGN is expected to produce strong outflows, in particular a UV-line driven wind. Despite providing a good fit to the data, current spectral models of the X-ray spectrum of AGN observed through an accretion disk wind are ad-hoc in their treatment of the properties of the wind material. In order to address these limitations we adopt a numerical computation method that links a series of radiative transfer calculations, incorporating the effect of a global velocity field in a self-consistent manner (XSCORT). We present a series of example spectra from the XSCORT code that allow us to examine the shape of AGN X-ray spectra seen through a wind, for a range of velocity and density distributions, total column densities and initial ionization parameters. These detailed spectral models clearly show considerable complexity and structure that is strongly affected by all these factors. The presence of sharp features in the XSCORT spectra contrasts strongly with both the previous models and with the smooth nature of the observed X-ray spectra of AGN with soft X-ray excesses, demonstrating that accretion disk winds are unlikely to be the origin of this mysterious spectral feature. The most significant parameter affecting the presence of the sharp features in the models is the terminal velocity of the wind. Increasing the terminal velocity of the absorbing material to ~c, and hence dramatically increasing the velocity dispersion across the wind, could potentially remove these features resulting in a spectrum similar to the previous models. Such a fast moving outflow cannot be associated with a radiatively driven accretion disk wind, however the presence of a highly relativistic jet may provide an origin for such material.