An exploratory study of the hard X-ray variability properties of PG   quasars with RXTE
  We have monitored with the RXTE PCA the variability pattern of the 2-20 keV flux in four PG quasars (QSOs) from the Laor et al. (1994) sample. Six observations of each target at regular intervals of 1 day were performed. The sample comprises objects with extreme values of Balmer line width (and hence soft X-ray steepness) and spans about one order of magnitude in luminosity. The most robust result is that the variability amplitude decreases as energy increases. Several options for a possible ultimate driver of the soft and hard X-ray variability, such as the influx rate of Comptonizing relativistic particles, instabilities in the accretion flow or the number of X-ray active sites, are consistent with our results. 
M. Guainazzi (1), F. Fiore (2), E. Giallongo (2), A. Laor (3), M.   Elvis(4), A. Siemiginowska(4) ((1) XMM-SOC, VILSPA, Spai...