Magnetar giant flare high-energy emission

TL;DR

This paper examines models explaining high-energy emission in magnetar giant flares, evaluating inverse Compton scattering and synchrotron radiation, and discusses observational prospects with Fermi.

Contribution

It provides a detailed analysis of the inverse Compton scattering model and proposes an alternative synchrotron radiation model for high-energy emission in magnetar flares.

Findings

Inverse Compton scattering model is consistent with current data.

Synchrotron radiation model is favored by RHESSI observations.

Future Fermi observations could help distinguish emission mechanisms.

Abstract

High energy ($> 250$ keV) emission has been detected persisting for several tens of seconds after the initial spike of magnetar giant flares. It has been conjectured that this emission might arise via inverse Compton scattering in a highly extended corona generated by super-Eddington outflows high up in the magnetosphere. In this paper we undertake a detailed examination of this model. We investigate the properties of the required scatterers, and whether the mechanism is consistent with the degree of pulsed emission observed in the tail of the giant flare. We conclude that the mechanism is consistent with current data, although the origin of the scattering population remains an open question. We propose an alternative picture in which the emission is closer to that star and is dominated by synchrotron radiation. The $RHESSI$ observations of the December 2004 flare modestly favor this latter picture. We assess the prospects for the Fermi Gamma-Ray Space Telescope to detect and characterize a similar high energy component in a future giant flare. Such a detection should help to resolve some of the outstanding issues.