ROSAT observations of warm absorbers in AGN
  We study the properties of warm absorbers in several active galaxies (AGN) and present new candidates, using ROSAT X-ray observations. Several aspects of the characteristics of warm absorbers are discussed, constraints on the density and location of the ionized material are provided; the impact of the presence of the warm gas on other observable spectral regions is investigated, particularly with respect to the possibility of a warm-absorber origin of one of the known high-ionization emission-line region in AGN; the possibility of dust mixed with the warm material is critically assessed; and the thermal stability of the ionized gas is examined. Based on the properties of known warm absorbers, we then address the question of where else ionized absorbers might play a role in determining the traits of a class of objects or individual peculiar objects. In this context, the possibility to produce the steep soft X-ray spectra of narrow-line Seyfert-1 galaxies by warm absorption is explored, as compared to the alternative of an accretion-produced soft excess. The potentiality of explaining the strong spectral variability in RXJ0134-42 and the drastic flux variability in NGC 5905 via warm absorption is scrutinized. 
Stefanie Komossa, Henner Fink (MPE Garching, D)