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

Giant γ-ray flares comprise the most extreme radiation events observed from magnetars. Developing on (sub)millisecond time-scales and generating vast amounts of energy within a fraction of a second, the initial phase of these extraordinary bursts presents 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 γ-ray flares. Our main constraints are given by the observed emission time-scales, 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. ∼10^-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 time-scale may be equated with the growth time of a magnetohydrodynamic instability.