The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanism

TL;DR

This paper evaluates whether the linear tearing instability in a twisted magnetosphere can explain the rapid onset of giant magnetar flares, focusing on growth timescales and magnetic field constraints.

Contribution

It provides a critical analysis of the linear tearing mode as a trigger mechanism, linking growth timescales to observed flare rise times and constraining current sheet geometry.

Findings

Linear growth time is about 0.1 ms, matching observed flare rise times.

Constraints on current sheet geometry derived from growth time analysis.

Discussion on the validity of equating emission timescale with instability growth time.

Abstract

Giant $\gamma$-ray flares comprise the most extreme radiation events observed from magnetars. Developing on (sub)millisecond timescales and generating vast amounts of energy within a fraction of a second, the initial phase of these extraordinary bursts present a significant challenge for candidate trigger mechanisms. Here we assess and critically analyse the linear growth of the relativistic tearing instability in a globally twisted magnetosphere as the trigger mechanism for giant $\gamma$-ray flares. Our main constraints are given by the observed emission timescales, the energy output of the giant flare spike, and inferred dipolar magnetic field strengths. We find that the minimum growth time of the linear mode is comparable to the $e$-folding rise time, i.e. $\sim10^{-1}$ ms. With this result we constrain basic geometric parameters of the current sheet. We also discuss the validity of the presumption that the $e$-folding emission timescale may be equated with the growth time of an MHD instability.