Magnetar giant flare high-energy emission

High-energy (>250 keV) emission has been detected persisting forseveral tens of seconds after the initial spike of magnetar giant flares(GFs). It has been conjectured that this emission might arise viainverse Compton scattering in a highly extended corona generated bysuper-Eddington outflows high up in the magnetosphere. In this paper, weundertake a detailed examination of this model. We investigate theproperties of the required scatterers, and whether the mechanism isconsistent with the degree of pulsed emission observed in the tail ofthe GF. We conclude that the mechanism is consistent with current data,although the origin of the scattering population remains an openquestion. We propose an alternative picture in which the emission iscloser to that star and is dominated by synchrotron radiation. TheReuven Ramaty High Energy Solar Spectroscopic Imager observations of the2004 December flare modestly favour this latter picture. We assess theprospects for the Fermi Gamma-ray Space Telescope to detect andcharacterize a similar high-energy component in a future GF. Such adetection should help to resolve some of the outstanding issues.