XMM-Newton monitoring of X-ray variability in the quasar PKS 0558-504
  We present the temporal analysis of X-ray observations of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558-504 obtained during the XMM-Newton Calibration and Performance Verification (Cal/PV) phase. The long term light curve is characterized by persistent variability with a clear tendency for the X-ray continuum to harden when the count rate increases. Another strong correlation on long time scales has been found between the variability in the hard band and the total flux. On shorter time scales the most relevant result is the presence of smooth modulations, with characteristic time of ~ 2 hours observed in each individual observation. The short term spectral variability turns out to be rather complex but can be described by a well defined pattern in the hardness ratio-count rate plane. 
M. Gliozzi (1), W. Brinkmann (1), P.T. O'Brien (2), J.N. Reeves (2),   K.A. Pounds (2), M. Trifoglio (3), F. Gianotti (3) ((...