High-energy afterglow emission from giant flares of soft gamma-ray repeaters: the case of the 2004 December 27 event from SGR 1806–20

We discuss the high enegry afterglow emission (including high energy photons, neutrinos and cosmic rays) following the 2004 December 27 Giant Flare from SGR 1806-20. If the initial outflow is relativistic with a bulk Lorentz factor Γ0 ∼ tens, the high-energy tail of the synchrotron emission from electrons in the forward shock region gives rise to a prominent sub-GeV emission, if the electron spectrum is hard enough and if the intial Lorentz factor is high enough. This signal could serve as a diagnosis of the initial Lorentz factor of the giant flare outflow. This component is potentially detectable by GLAST if a similar giant flare occurs in the GLAST era. With the available 10 MeV data, we constrain that Γ0 < 50 if the electron distribution is a single power law. For a broken power law distribution of electrons, a higher Γ0 is allowed. At energies higher than 1 GeV, the flux is lower because of a high energy cut off of the synchrotron emission component. The synchrotron self-Compton emission component and the inverse Compton scattering component off the photons in the giant flare oscillation tail are also considered, but they are found not significant given a moderate Γ0 (e.g. ≤ 10). The forward shock also accelerates cosmic rays to the maximum energy 10eV, and generate neutrinos with a typical energy 10eV through photomeson interaction with the X-ray tail photons. However, they are too weak to be detectable.