  On December 27, 2004, plasma particle detectors on the GEOTAIL spacecraft
detected an extremely strong signal of hard X-ray photons from the giant flare
of SGR1806-20, a magnetar candidate. While practically all gamma-ray detectors
on any satellites were saturated during the first ~500 ms interval after the
onset, one of the particle detectors on GEOTAIL was not saturated and provided
unique measurements of the hard X-ray intensity and the profile for the first
600 ms interval with 5.48 ms time resolution. After ~50 ms from the initial
rapid onset, the peak photon flux (integrated above ~50 keV) reached the order
of 10^7 photons sec^{-1} cm^{-2}. Assuming a blackbody spectrum with kT=175
keV, we estimate the peak energy flux to be 21 erg sec^{-1} cm^{-2} and the
fluence (for 0-600 ms) to be 2.4 erg cm^{-2}. The implied energy release
comparable to the magnetic energy stored in a magnetar (~10^{47} erg) suggests
an extremely efficient energy release mechanism.
