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

• Published in: Publications of the Astronomical Society of Japan 2007-06, Vol.59 (3), p.653-678
• Main Authors: Nakagawa, Yujin E., Yoshida, Atsumasa, Hurley, Kevin, Atteia, Jean-Luc, Maetou, Miki, Tamagawa, Toru, Suzuki, Motoko, Yamazaki, Tohru, Tanaka, Kaoru, Kawai, Nobuyuki, Shirasaki, Yuji, Pelangeon, Alexandre, Matsuoka, Masaru, Vanderspek, Roland, Crew, Geoff B., Villasenor, Joel S., Sato, Rie, Sugita, Satoshi, Kotoku, Jun’ichi, Arimoto, Makoto, Pizzichini, Graziella, Doty, John P., Ricker, George R.
• Format: Article
• Language: English
• Subjects: Astrophysics; Cosmology and Extra-Galactic Astrophysics; Physics
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/59.3.653

Temporal and spectral studies of short bursts ( $\lesssim$ 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 $\pm$ 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. From the data, the spectral evolution during bursts with a time scale of $\gtrsim$ 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 $\sim 4$ and $\sim 11$ keV. From a timing analysis to 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 a 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.