David Eichler: A model for the large amplitude QPO luminosity variation in the tail of SGR giant flares

We heard some discussion earlier in the session about the seismic vibrations that have been detected in the aftermath of giant flares from magnetars. One of the big questions relating to the oscillations (which allow us to do seismology and study the interior of the stars) is how a vibration of the stellar surface can generate varying X-ray emission. Particularly challenging for theorists are the high amplitudes of the variations in the X-ray emission. These are far too large to be explained by physical motions of the star's crust (it would be ripped apart) - so you need some kind of amplification mechanism. David Eichler presented a new model that may resolve this problem. The model relies on the fact that torsional oscillations of the crust (twisting motions) will also force the magnetic field to twist and oscillate. The associated currents drive variations in density; resonant cyclotron upscattering then operates, with varying optical depth, to generate high amplitude variable X-ray emission. The nice thing about the model is that you don't need large amplitude crust movements to get much larger amplitude variations in the X-ray. David also pointed out that the energy deposited in the crust as the oscillations dissipate energy could be responsible for the observed afterglows.