The impact of accretion disc winds on the X-ray spectrum of AGN - I. XSCORT

Abstract

The accretion disc in active galactic nucleus (AGN) is expected to produce strong outflows, in particular an ultraviolet (UV)-line-driven wind. Several observed spectral features, including the soft X-ray excess, have been associated with the accretion disc wind. However, current spectral models of the X-ray spectrum of AGN observed through an accretion disc wind, known to provide a good fit to the observed X-ray data, are ad hoc in their treatment of the outflow velocity and density of the wind material. In order to address these limitations we adopt a numerical computational method that links a series of radiative transfer calculations, incorporating the effect of a global velocity field in a self-consistent manner {XSTAR Simulation Chain for Outflows with Radiative Transfer (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 smooth wind with terminal velocity of 0.3c, as appropriate for a UV-line-driven wind. We calculate spectra for a range of different acceleration laws, density distributions, total column densities and ionization parameters, but all these have sharp features that contrast strongly with both the previous `smeared absorption' models, and with the observed smoothness of the soft X-ray excess. This rules out absorption in a radiatively driven accretion disc wind as the origin of the soft X-ray excess, though a larger terminal velocity, possibly associated with material in a magnetically driven outflow/jet, may allow outflow models to recover a smooth excess.