A comprehensive study of short bursts from SGR 1806-20 and SGR1900+14 detected by HETE-2

Abstract

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 SGR 1806-20 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 ∼ 11 keV. From a timing analysis to the SGR 1806-20 data, a time lag of 2.2±0.4 ms 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 ≲ 700 km) emission region of a softer component (say, ∼ 4keV), which could be reprocessed X-rays by higher energy (≳ 11 keV) photons from an emission region near the stellar surface. © 2007. Astronomical Society of Japan.