Fermi Gamma-ray Space Telescope

Magnetar giant flare timescales and the relativistic tearing mode

Chris Elenbaas
Anna L. Watts

Abstract:

Transient giant $gamma$-ray flares comprise the most extreme radiation events to have been observed from magnetars. Developing on (sub)millisecond timescales and expelling vast amounts of energy within a mere fraction of a second, the initial phase of these extraordinary bursts present a significant challenge for candidate trigger mechanisms. Here we revise and critically analyse the growth of the relativistic tearing instability in a global magnetospheric current sheet as the trigger mechanism for giant $gamma$-ray flares. Our main constraints are given by the observed emission timescales [$e$-folding rise time, peak time (time from flare onset until photon flux peak), and spike time (duration of the initial spike)], the energy output of the giant flare spike, and inferred dipolar magnetic field strengths. From pressure balance and energy conservation considerations we derive a typical current sheet thickness $deltasim10^4$ cm and height of the reconnection region $rsim10^7$ cm, which results in an average reconnection speed (rate at which field lines are advected into the diffusion region) $sim10^{-3}c$ for the three observed giant flares. We further discuss the viability of the assumption that the $e$-folding emission timescale may be equated with the growth time of an MHD instability; it is not self-evident that the radiation would be generated and escape for the system simultaneously as the instability develops.