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We present the unsaturated peak profile of SGR 1900+14 giant flare on 1998August 27. This was obtained by particle counters of the Low Energy Particleinstrument onboard the GEOTAIL spacecraft. The observed peak profile revealedfour characteristic structures: initial steep rise, intermediate rise to thepeak, exponential decay and small hump in the decay phase. From this lightcurve, we found that the isotropic peak luminosity was $2.3\times10^{46}$ ergs$^{-1}$ and the total energy was $4.3 \times 10^{44}$ erg s$^{-1}$ ($E\gtrsim$50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that thespectrum is optically thin thermal bremsstrahlung with $kT =$ 240 keV. Theseare consistent with the previously reported lower limits derived from Ulyssesand Konus-Wind observations. A comparative study of the initial spikes of SGR1900+14 giant flare in 1998 and SGR 1806-20 in 2004 is also presented. Thetimescale of the initial steep rise shows the magnetospheric origin, while thetimescale of the intermediate rise to the peak indicates that it originatesfrom the crustal fracturing. Finally, we argue that the four structures andtheir corresponding timescales provide a clue to identify extragalactic SGRgiant flares among short GRBs.

astrophysical journal/the astrophysical journal

Comparative study of the inial spikes of SGR giant flares in 1998 and  2004 observed with GEOTAIL: Do magnetospheric instabilities trigger large  scale fracturing of magnetar's crust?