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 a nd the survey data of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PK S 0558-504. We find strong and persistent X-ray variability on both short a nd medium time-scales. The most extreme amplitude variations require a radi ative efficiency exceeding the theoretical maximum for a Schwarzschild blac k hole, suggesting the presence of a rotating black hole or the influence o f relativistic beaming effects. The spatial analysis rules out the possibil ity that the high luminosity and the strong variability are related to a ne arby source.