Abstract

Recent observations with ROSAT have revealed an absorption feature in the spectra of some active galaxies, which is associated with the K–edge of highly ionized oxygen. This confirms that a large column of partially ionized material, the so–called warm absorber, which had been inferred from previous X–ray observations at higher energies, is present in the line–of–sight.

Introduction

X–ray evidence for highly ionized material in the line–of–sight to AGN has been accumulating over the past few years. Halpern (1984) originally suggested that highly ionized gas might be responsible for the soft X–ray absorption observed in some AGN, and cause spectral variations, as the opacity of the material changes with the ionizing flux. Further evidence was found using EXOSAT with many workers presenting evidence for flux–correlated changes in the soft X–ray absorption (e.g., Pan et al. 1990). However, given the widely recognized complexity of Seyfert spectra and the many candidate processes for producing the spectral changes, such observations were inconclusive. Subsequently, analysis of the Ginga spectra showed evidence for absorption at the iron–K edge, at an energy apparently higher than that expected from cold iron at 7.1 keV, in ∼ 50% of a sample of Seyfert galaxies. Nandra (1991) found a mean energy ∼ 8 – 8.5 keV, depending on the assumed continuum model. Again, however, it was difficult to establish the ionization state, or even the significance of the edge feature, in many individual cases, given the modest resolution of the LAC detector.