Recent ROSAT observations have established the presence of ionized matter along the line of sight in active galactic nuclei (the warm absorber). Here we investigate the iron Kalpha resonant absorption from the ionized matter, and find that the total equivalent width could be very relevant, up to several tens of eV. We point out its importance in determining the physical status and the geometry of the warm absorber. On the other hand, we warn that it can confuse, if neglected, the measurement of the broad iron line emitted by the accretion disc.
Matt, G. (1994). IRON K-ALPHA RESONANT ABSORPTION IN WARM ABSORBERS AROUND ACTIVE GALACTIC NUCLEI. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 267(3), L17-L20.