On the trigger mechanisms for SGR giant flares

TL;DR

This paper investigates two potential trigger mechanisms for SGR giant flares, one internal involving magnetic field instabilities and crust heating, and the other external involving magnetospheric reconnection, linking flare initiation to precursor activity.

Contribution

It introduces a novel internal magnetic instability model and a reconnection-based external model, explaining flare triggers and associated phenomena like precursors and afterglows.

Findings

Internal magnetic instability timescale matches quiescent period

Reconnection model explains nonthermal radiation and afterglows

Both models link flare onset to precursor activity

Abstract

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 March 5, 1979, August 27, 1998, and December 27, 2004. The last two events were found to be much more powerful than the first, and both showcased the existence of a precursor, that 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 timescale 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 preflare quiescent time. This model naturally explains the origin of the observed nonthermal radiation during the flares, as well as the post flare radio afterglows.