Magnetar giant flares and afterglows as relativistic magnetized explosions

Abstract

We propose that giant flares on soft γ-ray repeaters produce relativistic, strongly magnetized, weakly baryon-loaded magnetic clouds, somewhat analogous to solar coronal mass ejection (CME) events. The flares are driven by unwinding of the internal non-potential magnetic field which leads to a slow build-up of magnetic energy outside of the neutron star. For large magnetospheric currents, corresponding to a large twist of the external magnetic field, the magnetosphere becomes dynamically unstable on the Alfvén crossing time-scale of the inner magnetosphere, tA ∼ R NS/c ∼30 μs. The dynamic instability leads to the formation of dissipative current sheets through the development of a tearing mode. The released magnetic energy results in the formation of a strongly magnetized, pair-loaded, quasi-spherically expanding flux rope, topologically connected by the magnetic field to the neutron star during the prompt flare emission. The expansion reaches large Lorentz factors, Γ ∼ 10-20, at distances r ~ 1-2 x 107 cm, where a leptophotonic load is lost. Beyond this radius plasma is strongly dominated by the magnetic field, though some baryon loading, with M