A Comprehensive Study of Short Bursts from SGR1806$-$20 and SGR1900$+$14 Detected by HETE-2

Temporal and spectral studies of short bursts (? a few hundred milliseconds) are presented for the soft gamma repeaters (SGRs) 1806-20 and 1900+14 using the HETE-2 samples. In five years from 2001 to 2005, HETE-2 localized 50 bursts from SGR1806-20 and 5 bursts from SGR1900+14. The cumulative number-intensity distribution of SGR1806-20 in the active year 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. From the data, the spectral evolution during bursts with a time scale of ? 20ms does not seem to be common in the HETE-2 sample. The spectra of all short bursts are well reproduced by a two blackbody function with temperatures of ? 4 and ? 11keV. From a timing analysis to the SGR1806-20 data, a time lag of 2.2±0.4ms is found between the 30-100keV and 2-10keV 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 ? 700km) emission region of a softer component (say, ? 4keV), which could be reprocessed X-rays by higher energy (? 11keV) photons from an emission region near the stellar surface.