The first giant flare from SGR 1806-20:: Observations using the anticoincidence shield of the spectrometer on INTEGRAL

A giant flare from the soft gamma-ray repeater SGR 1806 - 20 has been discovered with the International Gamma-Ray Astrophysics Laboratory ( INTEGRAL) on 2004 December 27 and detected by many other satellites. This tremendous outburst, the first one observed from this source, was a hundred times more powerful than the two giant flares previously observed from other soft gamma-ray repeaters (SGRs). The 50 ms resolution light curve obtained with the anticoincidence shield of the spectrometer on INTEGRAL (SPI), which provides a high effective area above greater than or similar to 80 keV, shows evidence of emission lasting about 1 hr after the start of the outburst. This component, which decays in time as similar to t(-0.85), could be the first detection of a hard X-ray afterglow associated with an SGR giant flare. The short (0.2 s) initial pulse was so strong that it saturated the detector for similar to 0.7 s, and its backscattered radiation from the Moon was detected 2.8 s later. The following similar to 400 s - long tail, modulated at the neutron star rotation period of 7.56 s, had a fluence of 2.6 x 10(-4) ergs cm(-2) above 80 keV, which, extrapolating to lower energies, corresponds to an emitted energy of 1.6 x 10(44) d(15kpc)(2) ergs at E > 3 keV. This is of the same order as that in the pulsating tails of the two giant flares seen from other SGRs, despite the hundredfold larger overall emitted energy of the 2004 December 27 event.