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 SGR 1900 $+$14. The cumulative number–intensity distribution of SGR 1806$-$20 in the active year 2004 is well described by a power-law model with an index of $-$1.1±0.6. It is consistent with previous studies, but burst data taken in other years clearly give a steeper distribution. This may suggest that more energetic bursts could occur more frequently in periods of greater activity. From the data, the spectral evolution during bursts with a time scale of 20 ms does not seem to be common in the HETE-2 sample. The spectra of all short bursts are well reproduced by a two blackbody function with temperatures of ~ 4 and ~ 11keV. From a timing analysis to the SGR 1806$-$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 a 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.