The Giant Flare of 2004 December 27 from SGR 1806−20

The giant flare of December 27, 2004 from SGR 1806-20 represents one of themost extraordinary events captured in over three decades of monitoring thegamma-ray sky. One measure of the intensity of the main peak is its effect onX- and gamma-ray instruments. RHESSI, an instrument designed to study thebrightest solar flares, was completely saturated for ~0.5 s following the startof the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at thetime of the giant flare, however, allowed RHESSI a unique view of the giantflare event, including the precursor, the main peak decay, and the pulsed tail.Since RHESSI was saturated during the main peak, we augment these observationswith Wind and RHESSI particle detector data in order to reconstruct the mainpeak as well. Here we present detailed spectral analysis and evolution of thegiant flare. We report the novel detection of a relatively soft fast peak justmilliseconds before the main peak, whose timescale and sizescale indicate amagnetospheric origin. We present the novel detection of emission extending upto 17 MeV immediately following the main peak, perhaps revealing ahighly-extended corona driven by the hyper-Eddington luminosities. The spectralevolution and pulse evolution during the tail are presented, demonstratingsignificant magnetospheric twist and evolution during this phase. Blackbodyradii are derived for every stage of the flare, which show remarkable agreementdespite the range of luminosities and temperatures covered. Finally, we placesignificant upper limits on afterglow emission in the hundreds of secondsfollowing the giant flare.Comment: 32 pages, 14 figures, submitted to Ap