We present the results of temporal and spectral studies of the short burst
(less than a few hundred milliseconds) from the soft gamma repeaters (SGRs)
1806-20 and 1900+14 using the HETE-2 samples. In five years from 2001 to 2005,
HETE-2 detected 50 bursts which were localized to SGR 1806-20 and 5 bursts
which were localized to SGR 1900+14. Especially SGR 1806-20 was active in 2004,
and HETE-2 localized 33 bursts in that year. The cumulative number-intensity
distribution of SGR 1806-20 in 2004 is well described by a power law model with
an index of -1.1+/-0.6. It is consistent with previous studies but burst data
taken in other years clearly give a steeper distribution. This may suggest that
more energetic bursts could occur more frequently in periods of greater
activity. A power law cumulative number-intensity distribution is also known
for earthquakes and solar flares. It may imply analogous triggering mechanisms.
Although spectral evolution during bursts with a time scale of > 20 ms is not
common in the HETE-2 sample, spectral softening due to the very rapid (< a few
milliseconds) energy reinjection and cooling may not be excluded. The spectra
of all short bursts are well reproduced by a two blackbody function (2BB) with
temperatures ~4 and ~11 keV. From the timing analysis of the SGR 1806-20 data,
a time lag of 2.2+/-0.4 ms is found between the 30-100 keV and 2-10 keV
radiation bands. This may imply (1) a very rapid spectral softening and energy
reinjection, (2) diffused (elongated) emission plasma along the magnetic field
lines in pseudo equilibrium with multi-temperatures, or (3) a separate (located
at < 700 km) emission region of softer component (say, ~4 keV) which could be
reprocessed X-rays by higher energy (> 11 keV) photons from an emission region
near the stellar surface.Comment: 50 pages, 14 figures, accepted for publication in PAS
