AGN Outflows: Analysis of the Absorption Troughs

With the advent of Chandra and XMM, X-ray lines spectroscopy of AGN outflows is off to an exciting start. In this paper we illuminate some of the complications involved in extracting the outflow's physical conditions (ionization equilibrium, total column density, chemical abundances) from such spectroscopic data. To do so we use the example provided by high-quality FUSE and HST observations of the outflow seen in NGC 985. We show how simple determinations of the column density in the UV absorption lines often severely underestimate the real column densities. This is due to strong non-black saturation of the absorption troughs, where in many cases the UV line profile mainly reflects the velocity-dependent covering factor rather than the column density distribution. We then show that underestimating individual ionic column densities by a factor of 5 can cause a two orders of magnitude error in the inferred total column density of the outflow. In some case this will be enough to associate the UV and X-ray absorber with the same outflowing material. Finally, we note that the UV spectra of NGC 985 have 10--20 times the resolution and 2--5$\times$ the S/N per resolution element compared with the best available Chandra spectra of similar objects. Therefore, in the X-ray band non-black saturation and velocity-dependent covering factor effects will only become abundantly clear using vastly more capable future X-ray telescopes. However, by taking into consideration the lessons learned from the UV band, we can greatly improve the quality of physical constraints we extract from current X-ray data of AGN outflows.