  On 2011 September 6, we observed an X2.1-class flare in continuum and
H$\alpha$ with a frame rate of about 30~Hz. After processing images of the
event by using a speckle-masking image reconstruction, we identified
white-light (WL) flare ribbons on opposite sides of the magnetic neutral line.
We derive the lightcurve decay times of the WL flare kernels at each resolution
element by assuming that the kernels consist of one or two components that
decay exponentially, starting from the peak time. As a result, 42% of the
pixels have two decay-time components with average decay times of 15.6 and 587
s, whereas the average decay time is 254 s for WL kernels with only one
decay-time component. The peak intensities of the shorter decay-time component
exhibit good spatial correlation with the WL intensity, whereas the peak
intensities of the long decay-time components tend to be larger in the early
phase of the flare at the inner part of the flare ribbons, close to the
magnetic neutral line. The average intensity of the longer decay-time
components is 1.78 times higher than that of the shorter decay-time components.
If the shorter decay time is determined by either the chromospheric cooling
time or the nonthermal ionization timescale and the longer decay time is
attributed to the coronal cooling time, this result suggests that WL sources
from both regions appear in 42% of the WL kernels and that WL emission of the
coronal origin is sometimes stronger than that of chromospheric origin.
