  We present soft X-ray variability time-scales for 65 active galactic nuclei
(AGNs) derived from ROSAT/PSPC pointing data. For these 65 objects with obvious
exponential increasing or decreasing patterns in their light curves, we use the
exponential time-scales and find they are more suitable for analyzing ROSAT
light curve data. We also discuss some physical constraints on the central
engine implied by our results. Assuming that this soft X-ray variability
exponential time-scale is approximately equal to the thermal timescale of the
standard accretion disk, we obtain the accretion rate, the size of soft x-ray
radiation region and the compactness parameter for 37 AGNs, using their
recently estimated central black hole masses. For 12 of these 37 AGNs, the
radii of the gravitational instability in the standard thin accretion disks are
obtained using central black hole masses and the calculated accretion rates.
These are consistent with the results from the reverberation mapping method.
These results provide supporting evidence that such gravitational instabilities
contribute to the formation of the Broad Line Regions (BLRs) in AGNs.
