The highly ionised absorbing material in MR2251-178

Ginga observations have suggested the presence of iron absorption edges at energies appropriate for highly ionised material in the X-ray spectra of a significant number of active galactic nuclei. Here, we present a detailed self-consistent analysis of a series of Ginga and EXOSAT observations of MR2251-178, the original prototype of this warm-absorber phenomenon. The observations do indeed support the presence of a substantial column density of highly ionised material.

The ionisation state of the material responds to changes in the the luminosity of the ionising continuum, with the ionisation state of oxygen being particularly sensitive. Analysis of this variability leads to constraints on the density and location of this material as well as its total amount.