Yuri Lyubarsky: Theoretical overview of magnetars

The strong magnetic fields (~10^15 G) in magnetars feed many types of activity, and the emission from magnetars is powered by the field rather than by rotation or accretion. By restricting electron motion, the magnetic fields also suppress the radiation cross-section, thus allowing large super-Eddington luminocities.

A 10^46 erg giant flare was seen from the magnetar SGR 1806-20 on Dec. 27th 2004, which provides a useful test of magnetar theory. Despite being ~100 times brighter than other flares from this source, the energy in the pulsating tail was similar to that of other flares. This is thought to be because the energy in the tail is set by the storage capacity in the magnetosphere, which mostly depends on the magnetic field. Mass ejection was also detected in connection with the giant flare, by resolving the ejected cloud using radio interferometry. Further, QPOs were detected in the tail of the giant flare.

QPOs open for the possibility of NS seismology. Intermittent QPOs can probe (perhaps) shear modes in the NS crust, while low frequency (~20Hz) QPOs can probe the Alfven speed in the core. Magnetic fields couple the crust to the core on timescales of 0.01-0.1s, and this must be considered when interpreting QPOs.

The trapped fireball model predicts frequency dependent radiation cross-section, and this produes a flat spectrum below BB peak. One should attempt to fit this to the spectrum of sources which have previously been fitted with a BB plus power law spectrum.

The magnetar paradigm for SGRs and AXPs is now unquestioned, and high quality spectral data require sophisticated models of the radiation transfer. X-ray polarimitry would be a great help, but is presently unavailable.