A QED Model for the Origin of Bursts from Soft Gamma Repeaters and Anomalous X-Ray Pulsars

We propose a model to account for the bursts from soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) in which quantum electrodynamics plays a vital role. In our theory, which 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_NS>~B_QED~4.4×10¹³ G, the interaction of the wave fields with the vacuum produces nonlinearities 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 γ-rays, with a spectrum similar to those seen in bursts from SGRs and AXPs. In addition, the mostly thermal radiation will be accompanied by a high-energy tail of synchrotron emission, whose existence can be used to test this theory. Our model also predicts that for disturbances with a given wavelength and amplitude, only stars with magnetic fields above a critical threshold will experience fast-mode breakdown in their magnetospheres. In principle, this distinction provides an explanation for why SGRs and AXPs exhibit burst activity while high-field radio pulsars apparently do not.