The first giant flare from SGR 1806 – 20 : observations with the INTEGRAL SPI Anti - Coincidence Shield

A giant flare from the Soft Gamma-ray Repeater SGR 1806–20 has been discovered with the INTEGRAL gamma-ray observatory 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 (SGR). The 50 ms resolution light curve obtained with the Anticoincidence Shield of the INTEGRAL spectrometer SPI, which provides a high effective area above > ∼ 80 keV, shows evidence for emission lasting about one hour after the start of the outburst. This component, which decays in time as ∼t −0.85 , could be the first detection of a hard X-ray afterglow associated to an SGR giant flare. The short (0.2 s) initial pulse was so strong to saturate the detector for ∼0.7 s, and its backscattered radiation from the Moon was detected 2.8 s later. The following ∼400 s long tail, modulated at the neutron star rotation period of 7.56 s, had a fluence of 2.6×10 −4 erg cm −2 above 80 keV, which extrapolating to lower energies corresponds to an emitted energy of 1.6×10 44 d 2 15kpc erg at E>3 keV. This is of the same order of 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.