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We analyze the data obtained when the Konus-Wind gamma-ray spectrometerdetected a giant flare in SGR 1806-20 on December 27, 2004. The flare issimilar in appearance to the two known flares in SGR 0526-66 and SGR 1900+14while exceeding them significantly in intensity. The enormous X-ray andgamma-ray flux in the narrow initial pulse of the flare leads to almostinstantaneous deep saturation of the gamma-ray detectors, ruling out thepossibility of directly measuring the intensity, time profile, and energyspectrum of the initial pulse. In this situation, the detection of anattenuated signal of Compton back-scattering of the initial pulse emission bythe Moon with the Helicon gamma-ray spectrometer onboard the Coronas-Fsatellite was an extremely favorable circumstance. Analysis of this signal hasyielded the most reliable temporal, energy, and spectral characteristics of thepulse. The temporal and spectral characteristics of the pulsating flare tailhave been determined from Konus-Wind data. Its soft spectra have been found tocontain also a hard power-law component extending to 10 MeV. A weak afterglowof SGR 1806-20 decaying over several hours is traceable up to 1 MeV. We alsoconsider the overall picture of activity of SGR 1806-20 in the emission ofrecurrent bursts before and after the giant flare.Comment: 29 pages including 18 figures; to appear in Astronomy Letters, 2007,  33, p 1-1

Giant flare in SGR 1806-20 and its Compton reflection from the Moon