The variability properties of X-ray-steep and X-ray-flat quasars

We have studied the variability of six low-redshift, radio-quiet PG quasars on three timescales (days, weeks, and months) using the ROSAT HRI. The qua sars were chosen to lie at the two extreme ends of the ROSAT PSPC spectral index distribution and hence also of the H beta FWHM distribution. The obse rvation strategy has been carefully designed to provide even sampling on th ese three basic timescales and to provide a uniform sampling among the quas ars. We have found clear evidence that the X-ray-steep, narrow-H beta quasa rs systematically show larger amplitude variations than the X-ray-flat, bro ad-H beta quasars on timescales from 2 to 20 days. On longer timescales, we do not fmd significant differences between steep and flat quasars, althoug h the statistics are poorer. We suggest that the above correlation between variability properties and spectral steepness can be explained by a scenari o in which the X-ray-steep, narrow-line objects are in a higher L/L-Edd sta te with respect to the X-ray-flat, broad-line objects. We evaluated the pow er spectrum of PG 1440+356 (the brightest quasar in our sample) between 2 x 10(-7) and similar to 10-3 Hz, where it goes into the noise. The power spe ctrum is roughly consistent with a 1/f law between 10(-3) and 2 x 10(-6) Hz . Below this frequency it flattens significantly.