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 quasars 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 observation strategy has been carefully designed to provide even sampling on these three basic timescales and to provide a uniform sampling among the quasars. We have found clear evidence that the X-ray-steep, narrow-H beta quasars systematically show larger amplitude variations than the X-ray-flat, broad-H beta quasars on timescales from 2 to 20 days. On longer timescales, we do not fmd significant differences between steep and flat quasars, although the statistics are poorer. We suggest that the above correlation between variability properties and spectral steepness can be explained by a scenario in which the X-ray-steep, narrow-line objects are in a higher L/L-Edd state with respect to the X-ray-flat, broad-line objects. We evaluated the power 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 spectrum is roughly consistent with a 1/f law between 10(-3) and 2 x 10(-6) Hz. Below this frequency it flattens significantly.