A QED Model for the Origin of Bursts from SGRs and AXPs

Abstract

We propose a model to account for the bursts from soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) in which quantum electrodynamics (QED) plays a vital role. In our theory, that we term ``fast-mode breakdown,'' magnetohydrodynamic (MHD) waves that are generated near the surface of a neutron star and propagate outward through the magnetosphere will be modified by the polarization of the vacuum. For neutron star magnetic fields $B_\rmscr{NS} \gtrsim B_\rmscr{QED} \approx 4.4 \times 10^{13}$ G, the interaction of the wave fields with the vacuum produces non-linearities in fast MHD waves that can steepen in a manner akin to the growth of hydrodynamic shocks. Under certain conditions, fast modes can develop field discontinuities on scales comparable to an electron Compton wavelength, at which point the wave energy will be dissipated through electron-positron pair production. We show that this process operates if the magnetic field of the neutron star is sufficiently strong and the ratio of the wavelength of the fast mode to its amplitude is sufficiently small, in which case the wave energy will be efficiently converted into an extended pair-plasma fireball. The radiative output from this fireball will consist of hard X-rays and soft $\gamma$-rays, with a spectrum similar to those seen in bursts from SGRs and AXPs (abridged).