1. A Comprehensive Study of Short Bursts from SGR 1806-20 and SGR 1900+14 Detected by HETE-2
- Author
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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., and Ricker, George R.
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Astrophysics - Abstract
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+/-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 > 20 ms is not common in the HETE-2 sample, spectral softening due to the very rapid (< 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 ~4 and ~11 keV. From the timing analysis of 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 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., Comment: 50 pages, 14 figures, accepted for publication in PASJ
- Published
- 2007
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