High Resolution X-ray Spectroscopy and the Nature of Warm Absorbers in   AGN
  Although soft X-ray absorption features in AGN were discovered almost ten years ago, the nature and location of the gas creating them has remained controversial. However, by making use of the newly-available high-resolution spectra provided by XMM-Newton and Chandra, we should be able to make substantial advances. The first such spectra indicate that multiple ionization states coexist in the absorber; this is a natural consequence of photoionization physics. Photoionized evaporation in the presence of a copious mass source locks the ratio of ionizing intensity to pressure to a critical value. A broad range of temperatures can all coexist in equilibrium for this value of the ratio of ionizing intensity to pressure. Consequently, the flow is expected to be strongly inhomogeneous in temperature. The inferred distance of this material from the source of ionizing radiation depends on how much matter exists at the highest-obtainable temperature. This distance can be measured by monitoring how ionic column densities respond to changes in the ionizing continuum on timescales of days to years. 
Julian H. Krolik