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Giant $\gamma$-ray flares comprise the most extreme radiation events observedfrom magnetars. Developing on (sub)millisecond timescales and generating vastamounts of energy within a fraction of a second, the initial phase of theseextraordinary bursts present a significant challenge for candidate triggermechanisms. Here we assess and critically analyse the linear growth of therelativistic tearing instability in a globally twisted magnetosphere as thetrigger mechanism for giant $\gamma$-ray flares. Our main constraints are givenby the observed emission timescales, the energy output of the giant flarespike, and inferred dipolar magnetic field strengths. We find that the minimumgrowth 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 ofthe 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 MHDinstability.

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