ROSAT HRI monitoring of X-ray variability in the NLS1 galaxy PKS   0558-504
  We present the results of ROSAT High Resolution Imager (HRI) observations and the survey data of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558--504. We find strong and persistent X-ray variability on both short and medium time-scales. The most extreme amplitude variations require a radiative efficiency exceeding the theoretical maximum for a Schwarzschild black hole, suggesting the presence of a rotating black hole or the influence of relativistic beaming effects. The spatial analysis rules out the possibility that the high luminosity and the strong variability are related to a nearby source. 
M. Gliozzi (1), Th. Boller (1), W. Brinkmann (1), W.N. Brandt (2) ((1)   MPE Garching, (2) Pennsylvania State University, USA)