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 obs ervations of each target at regular intervals of 1 day were performed. The sample comprises objects with extreme values of Balmer line width (and henc e soft X-ray steepness) and spans about one order of magnitude in luminosit y, The most robust result is that the variability amplitude decreases as en ergy increases. Several options for a possible ultimate driver of the soft and hard X-ray variability, such as the influx rate of Comptonizing relativ istic particles, instabilities in the accretion flow or the number of X-ray 'active sites', are consistent with our results. (C) 2000 Elsevier Science B.V. All rights reserved.