2000A&A...354..411K

We present a study of the X-ray variability properties and spectral shapes of five active galaxies all of which show extreme or enigmatic X-ray properties. We focus on, 134, and NGC 4051, and briefly comment on Mrk 1298 and c. The individual objects were originally partly selected as candidates to host warm absorbers on the basis of (i) characteristic X-ray absorption features (NGC 4051, Mrk 1298, c), (ii) extreme X-ray spectral steepness ( ; this object is found to be located in the `zone of avoidance' when plotted in the Γ_x-FWHM_Hβ diagram), and (iii) drastic spectral variability (134). The temporal analysis reveals large-amplitude variability by a factor ∼30 in the long-term X-ray lightcurve of NGC 4051, very rapid variability of Mrk 1298, constant X-ray flux of the NLSy1 galaxy, and constant mean countrate of 134 despite huge spectral changes. Besides the warm absorber, several further mechanisms and their merits/shortcomings are investigated to explain the spectral characteristics of the individual objects. Different models are favored for different sources. Consequences for Narrow-line Seyfert 1s in general are discussed and we present results from photoionization models to distinguish between different suggested NLSy1 scenarios.