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-energy afterglow emission (including high-energy photons, neutrinos and cosmic rays) following the 2004 December 27 giant flare from the soft gamma-ray repeater (SGR) 1806-20. If the initial outflow is relativistic with a bulk Lorentz factor Gamma(0) similar to 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 initial 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 the Gamma-Ray Large Area Telescope (GLAST) if a similar giant flare occurs in the GLAST era. With the available 10-MeV data, we constrain that Gamma(0) < 50 if the electron distribution is a single power law. For a broken power-law distribution of electrons, a higher Gamma(0) is allowed. At energies higher than I 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 Gamma(0) (e.g. < 10). The forward shock also accelerates cosmic rays to the maximum energy 10(17) eV, and generates neutrinos with a typical energy 10(14) eV through photomeson interaction with the X-ray tail photons. However, they are too weak to be detectable.