Torsional Magnetic Oscillations in Type I X-Ray Bursts

Abstract

Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer of the star, $ΔR(t)$. The layer rotates slower than the star due to angular momentum conservation. The shear between the star and the layer acts to twist the star's dipole magnetic field giving at first a trailing spiral field. The twist of the field acts in turn to `torque up' the layer increasing its specific angular momentum. As the layer cools and contracts, its excess specific angular momentum causes it to {\it rotate faster} than the star which gives a leading spiral magnetic field. The process repeats, giving rise to torsional oscillations. We derive equations for the angular velocity and magnetic field of the layer taking into account the diffusivity and viscosity which are probably due to turbulence. The magnetic field causes a nonuniformity of the star's photosphere (at the top of the heated layer), and this gives rise to the observed X-ray oscillations. The fact that the layer periodically rotates faster than the star means that the X-ray oscillation frequency may ``overshoot'' the star's rotation frequency. Comparison of the theory is made with observations of Chakrabarty et al. (2003) of an X-ray burst of SAX J1808.4-3658.