  A statistic w, the differential coefficient of the mean absolute difference
of an observed lightcurve, is proposed for timescale analysis of shot widths.
The shortest width of random shots can be measured by the position of the lower
cut-off in the timescale spectrum of w. We use the statistic to analyze X-ray
lightcurves from a sample of neutron star and black hole binaries and the
results show that the timescale analysis can help us distinguish between
neutron star binaries and black hole binaries. The analysis can further reveal
the structure and dynamics of accretion disks around black holes.
