Abstract

Bright outbursts from Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are believed to be caused by instabilities in ultramagnetized neutron stars, powered by a decaying magnetic field. It was originally thought that these outbursts were due to reconnection instabilities in the magnetosphere, reached via slow evolution of magnetic footpoints anchored in the crust. Later models considered sudden shifts in the crust's structure. Recent observations of magnetars give evidence that at least some outburst episodes involve rearrangements and/or energy releases within the star. We suggest that bursting episodes in magnetars are episodes of rapid plastic yielding in the crust, which trigger “swarms” of reconnection instabilities in the magnetosphere. Magnetic energy always dominates; elastic energy released within the crust does not generate strong enough Alfvén waves to power outbursts. We discuss the physics of SGR giant flares, and describe recent observations that give useful constraints and clues.

Introduction: a neutron star's crust

The crust of a neutron star has several components: (1) a Fermi sea of relativistic electrons, which provides most of the pressure in the outer layers; (2) another Fermi sea of neutrons in a pairing-superfluid state, present only at depths below the “neutron drip” level where the mass-density exceeds ρdrip ≈ 4.6 × 1011 gm cm-3; and (3) an array of positively-charged nuclei, arranged in a solid (but probably not regular crystalline) lattice-like structure throughout much of the crust.