SIMBAD references

The variable warm absorber seen with ASCA in the X-ray spectrum of MCG-6-30-15 shows complex time behaviour in which the optical depth of Oviii anticorrelates with the flux whereas that of Ovii is unchanging. The explanation in terms of a two-zone absorber has since been challenged by BeppoSAX observations. These present a more complicated behaviour for the Oviii edge. We demonstrate here that the presence of a third, intermediate, zone can explain all the observations. In practice, warm absorbers are likely to be extended, multi-zone regions of which only part causes directly observable absorption edges at any given time.