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

Abstract

We propose a model to account for thebursts from soft gamma repeaters (SGRs) and anomalous X-ray pulsars(AXPs) in which quantum electrodynamics plays a vital role. In ourtheory, which we term ``fast-mode breakdown,'' magnetohydrodynamic(MHD) waves that are generated near the surface of a neutron starand propagate outward through the magnetosphere will be modified bythe polarization of the vacuum. For neutron star magnetic fieldsB NS ~B QED ~4.4�10 13 G, the interaction of the wave fieldswith the vacuum produces nonlinearities in fast MHD waves that cansteepen in a manner akin to the growth of hydrodynamic shocks. Undercertain conditions, fast modes can develop field discontinuitieson scales comparable to an electron Compton wavelength, at whichpoint the wave energy will be dissipated through electron-positronpair production. We show that this process operates if the magneticfield of the neutron star is sufficiently strong and the ratio of thewavelength of the fast mode to its amplitude is sufficiently small, inwhich case the wave energy will be efficiently converted into anextended pair plasma fireball. The radiative output from this fireballwill consist of hard X-rays and soft gamma-rays, with a spectrumsimilar to those seen in bursts from SGRs and AXPs. In addition, themostly thermal radiation will be accompanied by a high-energy tail ofsynchrotron emission, whose existence can be used to test this theory. Ourmodel also predicts that for disturbances with a given wavelength andamplitude, only stars with magnetic fields above a critical threshold willexperience fast-mode breakdown in their magnetospheres. In principle, thisdistinction provides an explanation for why SGRs and AXPs exhibitburst activity while high-field radio pulsars apparently do not.