The complex X-ray spectra of active galaxies with warm absorbers

Warm absorbers are an important new probe of the central regions of active galaxies. So far, they revealed their existence mainly in the soft X-ray spectral region. In observing and modeling this component, we can learn a lot about the nature of the warm absorber itself, its relation to other components of the active nucleus, and the intrinsic AGN X-ray spectral shape. Here, we briefly review the basic X-ray spectral features of warm absorbers (dust-free WAs, dusty WAs, peculiar ~1.1 keV absorption, emission and reflection components) and then discuss these in more detail based on analyses of individual objects observed with the X-ray satellite ROSAT. The importance of XMM in improving our knowledge of the warm material is discussed.