Publ. Astron. Soc. Jap., 59, 653-678 (2007/June-0)
A comprehensive study of short bursts from SGR1806-20 and SGR1900+14 detected by HETE-2.
NAKAGAWA Y.E., YOSHIDA A., HURLEY K., ATTEIA J.-L., MAETOU M., TAMAGAWA T., SUZUKI M., YAMAZAKI T., TANAKA K., KAWAI N., SHIRASAKI Y., PELANGEON A., MATSUOKA M., VANDERSPEK R., CREW G.B., VILLASENOR J.S., SATO R., SUGITA S., KOTOKU J., ARIMOTO M., PIZZICHINI G., DOTY J.P. and RICKER G.R.
Abstract (from CDS):
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 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 \gtrsim 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 \lesssim 700km) emission region of a softer component (say, ∼ 4keV), which could be reprocessed X-rays by higher energy (\gtrsim 11keV) photons from an emission region near the stellar surface.
gamma rays: observations - stars: pulsars: individual(SGR 1806-20, SGR 1900+14) - X-rays: stars
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