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 \pm 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 \gtrsim 20 ms is not common in the HETE-2 sample , spectral softening due to the very rapid ( \lesssim 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 \sim 4 and \sim 11 keV .
From the timing analysis of the SGR 1806 - 20 data , a time lag of 2.2 \pm 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 \lesssim 700 km ) emission region of softer component ( say , \sim 4 keV ) which could be reprocessed X-rays by higher energy ( \gtrsim 11 keV ) photons from an emission region near the stellar surface .