Crustal shear oscillations, magnetar spindown, and the 1998 August 27 flare

Abstract

Pure shear oscillations in neutron star crusts are strongly excited by magnetically-induced starquakes. These toroidal modes could play a key role in the physics of Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). Toroidal modes drive relativistic Alfven waves into the magnetosphere. Energy is lost along field lines that are blown open by this outflow pressure. This helps damp the crust vibrations while driving a relativistic wind of waves and particles away from the star. When channeled by a strong magnetic field, this outflow carries away angular momentum. Frequent small-scale fractures maintain a quasi-steady shear mode excitation in a SGR’s crust, accelerating SGR spindown. During energetic bursts or flares, large-scale fractures excite the vibrations to much higher amplitudes. Shear oscillations with energy ∼1044 E44 erg were probably excited during the 1998 August 27 event, comparable to the observed energy in hard photons. This inevitably drove transient accelerated spindown, with a ...