On the trigger mechanisms for soft gamma-ray repeater giant flares

We examine two trigger mechanisms, one internal and the other external to the neutron star, that give rise to the intense soft gamma-ray repeater (SGR) giant flares. So far, three giant flares have been observed from the three out of the seven confirmed SGRs on 1979 March 5, 1998 August 27 and 2004 December 27, respectively. The last two events were found to be much more powerful than the first, and both showcased the existence of a precursor, which we show to have had initiated the main flare. In the internal mechanism, we propose that the strongly wound-up poloidal magnetic field develops tangential discontinuities and dissipates its torsional energy in heating the crust. The time-scale for the instability to develop coincides with the duration of the quiescent state that followed the precursor. Alternatively, we develop a reconnection model based on the hypothesis that shearing motion of the footpoints causes the materialization of a Sweet-Parker current layer in the magnetosphere. The thinning of this macroscopic layer due to the development of an embedded super-hot turbulent current layer switches on the impulsive Hall reconnection, which powers the giant flare. Again, we show that the thinning time is on the order of the pre-flare quiescent time. This model naturally explains the origin of the observed non-thermal radiation during the flares as well as the post-flare radio afterglows.