GEOTAIL observation of the SGR1806-20 Giant Flare: The first 600 ms
Abstract
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 107 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 (~1047 erg) suggests an extremely efficient energy release mechanism.
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