On magnetar burst mechanisms

Neutron stars where the output emission is dominated by the decay of an ultra-strong magnetic field, typically in excess of 10¹⁴ Gauss, are characterized as magnetars. This decay may occur both gradually and abruptly, which in the latter case results in bright recurrent soft gamma-ray bursts and occasionally giant flares. Here we investigate and develop theoretical models of the trigger and emission mechanisms of these bursts, which involve extreme and fundamental (astro)physical processes. By comparing the model predictions with observed data, we aim to constrain distinct magnetar burst mechanisms. Assuming that magnetar giant flares are triggered by the spontaneous development of a tearing instability in a magnetospheric current sheet, we obtain estimates for the dimension of the corresponding reconnection region. Moreover, we assess inverse Compton and synchrotron emission models for high-energy emission that was observed following the impulsive phase of the giant flare from the magnetar SGR 1806-20 on 2004 December 27. We conclude that the high-energy emission may constitute synchrotron radiation from shock-accelerated particles, whereby the shock is generated between the evaporative outflow from the star and a downward directed reconnection exhaust. We furthermore study the characteristics of the recurrent bursts as manifestations of systems of self-organised critically, such as flares and crust-quakes, and develop a model where we investigate the detectability of a phase-dependence of magnetar burst properties.